Show nowarn / Wconf filters for a warning with @nowarn("verbose")

As already implemented in Scala 3

```
scala> @nowarn("v") def f = try 1
                            ^
       warning: A try without a catch or finally is equivalent to putting its body in a block; no exceptions are handled.
       Applicable -Wconf / @nowarn filters for this warning: msg=<part of the message>, cat=other, site=f
def f: Int
```

Author: lrytz

library/src/scala/annotation/nowarn.scala

@@ -14,15 +14,21 @@ package scala.annotation
 
 /** An annotation for local warning suppression.
   *
-  * The optional `value` parameter allows selectively silencing messages, see `scalac -Wconf:help`
-  * for help. Examples:
+  * The optional `value` parameter allows selectively silencing messages. See `-Wconf:help` for help
+  * writing a message filter expression, or use `@nowarn("verbose")` / `@nowarn("v")` to display message
+  * filters applicable to a specific warning.
+  *
+  * Examples:
   *
   * {{{
   *   def f = {
   *     1: @nowarn // don't warn "a pure expression does nothing in statement position"
   *     2
   *   }
   *
+  *   // show the warning, plus the applicable @nowarn / Wconf filters ("cat=other-pure-statement", ...)
+  *   @nowarn("v") def f = { 1; 2 }
+  *
   *   @nowarn def f = { 1; deprecated() } // don't warn
   *
   *   @nowarn("msg=pure expression does nothing")