Add reference docs for explain of find and aggregate (#626)

JAVA-3909

Author: jyemin

docs/reference/content/driver-reactive/tutorials/aggregation.md

@@ -93,3 +93,21 @@ collection.aggregate(
       )
 ).subscribe(new PrintDocumentSubscriber());
 ```
+
+### Explain an Aggregation
+
+To [explain]({{< docsref "reference/command/explain/" >}}) an aggregation pipeline, call the
+[`AggregatePublisher.explain()`]({{< apiref "mongodb-driver-reactivestreams" "com/mongodb/reactivestreams/client/AggregatePublisher.html#explain()" 
+> >}}) 
+method:
+
+```java
+collection.aggregate(
+      Arrays.asList(
+              Aggregates.match(Filters.eq("categories", "Bakery")),
+              Aggregates.group("$stars", Accumulators.sum("count", 1))))
+      .explain()
+      .subscribe(new PrintDocumentSubscriber());
+```
+
+The driver supports explain of aggregation pipelines starting with MongoDB 3.6.

docs/reference/content/driver-reactive/tutorials/perform-read-operations.md

@@ -170,6 +170,20 @@ collection.find(and(gte("stars", 2), lt("stars", 5), eq("categories", "Bakery"))
           .subscribe(new PrintDocumentSubscriber());
 ```
 
+### Explain
+
+To [explain]({{< docsref "reference/command/explain/" >}}) a find operation, call the
+[`FindPublisher.explain()`]({{< apiref "mongodb-driver-reactivestreams" "com/mongodb/reactivestreams/client/FindPublisher.html#explain()" >}}) 
+method:
+
+```java
+collection.find(and(gte("stars", 2), lt("stars", 5), eq("categories", "Bakery")))
+          .explain()
+          .subscribe(new PrintDocumentSubscriber());
+```
+
+The driver supports explain of find operations starting with MongoDB 3.0.
+
 ## Read Preference
 
 For read operations on [replica sets]({{<docsref "replication/" >}}) or [sharded clusters]({{<docsref "sharding/" >}}), applications can configure the [read preference]({{<docsref "reference/read-preference" >}}) at three levels:

docs/reference/content/driver-scala/tutorials/aggregation.md

@@ -86,3 +86,20 @@ collection.aggregate(
   )
 ).printResults()
 ```
+
+### Explain an Aggregation
+
+To [explain]({{< docsref "reference/command/explain/" >}}) an aggregation pipeline, call the
+[`AggregateObservable.explain()`]({{< apiref "mongo-scala-driver" "org/mongodb/scala/AggregateObservable.html#explain()" >}})
+method:
+
+```java
+collection.aggregate(
+      Arrays.asList(
+              Aggregates.match(Filters.eq("categories", "Bakery")),
+              Aggregates.group("$stars", Accumulators.sum("count", 1))))
+      .explain()
+      .printResults()
+```
+
+The driver supports explain of aggregation pipelines starting with MongoDB 3.6.

docs/reference/content/driver-scala/tutorials/perform-read-operations.md

@@ -156,6 +156,20 @@ collection.find(and(gte("stars", 2), lt("stars", 5), equal("categories", "Bakery
           .printResults()
 ```
 
+### Explain
+
+To [explain]({{< docsref "reference/command/explain/" >}}) a find operation, call the
+[`FindObservable.explain()`]({{< apiref "mongo-scala-driver" "org/mongodb/scala/FindObservable.html#explain()" >}})
+method:
+
+```scala
+collection.find(and(gte("stars", 2), lt("stars", 5), eq("categories", "Bakery")))
+          .explain()
+          .printResults()
+```
+
+The driver supports explain of find operations starting with MongoDB 3.0.
+
 ## Read Preference
 
 For read operations on [replica sets]({{<docsref "replication/" >}}) or [sharded clusters]({{<docsref "sharding/" >}}), applications can configure the [read preference]({{<docsref "reference/read-preference" >}}) at three levels:
@@ -240,7 +254,7 @@ For example, in the following, the `collWithReadConcern` instance has an AVAILAB
 
 ```scala
 val collWithReadConcern = collection.withReadConcern(ReadConcern.AVAILABLE)
-```
+```              
 
 You can build `MongoClientSettings`, `MongoDatabase`, or `MongoCollection` to include a combination of read concern, read preference, and [write concern]({{<docsref "reference/write-concern" >}}).
 

docs/reference/content/driver/tutorials/aggregation.md

@@ -100,3 +100,20 @@ collection.aggregate(
       )
 ).forEach(printBlock);
 ```
+
+### Explain an Aggregation
+
+To [explain]({{< docsref "reference/command/explain/" >}}) an aggregation pipeline, call the
+[`AggregateIterable.explain()`]({{< apiref "mongodb-driver-sync" "com/mongodb/client/AggregateIterable.html#explain()" >}}) method:
+
+```java
+Document explainResult = collection.aggregate(
+      Arrays.asList(
+              Aggregates.match(Filters.eq("categories", "Bakery")),
+              Aggregates.group("$stars", Accumulators.sum("count", 1))))
+      .explain();
+System.out.println(explainResult.toJson(JsonWriterSettings.builder().indent(true).build()));
+```
+
+The driver supports explain of aggregation pipelines starting with MongoDB 3.6.
+

docs/reference/content/driver/tutorials/perform-read-operations.md

@@ -178,6 +178,19 @@ collection.find(and(gte("stars", 2), lt("stars", 5), eq("categories", "Bakery"))
                 .forEach(printConsumer);
 ```
 
+
+### Explain
+
+To [explain]({{< docsref "reference/command/explain/" >}}) a find operation, call the 
+[`FindIterable.explain()`]({{< apiref "mongodb-driver-sync" "com/mongodb/client/FindIterable.html#explain()" >}}) method:
+
+```java
+Document explainResult = collection.find(and(gte("stars", 2), lt("stars", 5), eq("categories", "Bakery"))).explain();
+System.out.println(explainResult.toJson(JsonWriterSettings.builder().indent(true).build()));
+```
+
+The driver supports explain of find operations starting with MongoDB 3.0.
+
 ## MongoIterable
 
 The [`MongoIterable`]({{< apiref "mongodb-driver-sync" "com/mongodb/client/MongoIterable.html" >}}) interface provides helper methods to access the results of an operation: