feat(develop/span-first): Add more implementation guidelines and Dart example (#15754)

## DESCRIBE YOUR PR

Adds more implementation details to Span First and Dart code example

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---------

Co-authored-by: Ivana Kellyer <ivana.kellyer@sentry.io>

Author: buenaflor

develop-docs/sdk/telemetry/spans/implementation.mdx

@@ -48,7 +48,24 @@ If you're implementing Span-First (as a PoC) in your SDK, take an iterative appr
 To do: This section needs a few guidelines and implementation hints, including:
 - how to set a span active and remove it from the scope once it ends
 - languages having to deal with async context management
-- edge cases (e.g. adding a span with an explicit parent span that already ended)
+
+### `parentSpan` option for `startSpan`
+
+SDKs MUST expose a `parentSpan` option on the `startSpan` API which allows users to explicitly set the parent span of the new span.
+
+The `parentSpan` parameter has three distinct states: `undefined`, `null` and a span instance. See the [Span API documentation](../span-api) for the semantics.
+
+For languages that do not support an `undefined` state, SDKs SHOULD model this three-state behavior using platform-appropriate mechanisms. 
+Prefer solutions that preserve the semantic distinction between `undefined` and `null`, such as:
+
+- method/constructor overloading (e.g., an overload without `parentSpan`, and another accepting `parentSpan: Span?`),
+- a default sentinel value/object representing `undefined`,
+- or other idiomatic platform mechanisms (e.g., enum types).
+
+### Edge Cases
+
+- If `parentSpan` references a span that has already ended, the SDK SHOULD still create the new span and send it as a child of `parentSpan`.
+  - Handling and presentation of these relationships is deferred to downstream processing and the frontend/UI.
 
 ## Single-Span Processing Pipeline
 
@@ -119,12 +136,13 @@ Some rough pointers:
 - SDKs SHOULD follow one of the backend, mobile or browser telemetry buffer specifications. 
 - It is expected and fine to implement the proper, weighted buffering logic as a final step in the Span-First project. 
   Intermediate buffers MAY be simpler, for example disregard the priority logic and just buffer until a certain span length, size or time interval is reached.
+
 ## Release
 
-The initial PoC implementation of Span-First **SHOULD** be released in a **minor version** of the SDK.
+The initial PoC implementation of Span-First SHOULD be released in a minor version of the SDK.
 
-- This feature is entirely opt-in via `traceLifecycle = 'stream'` and therefore does **not** introduce breaking changes to existing users.
+- This feature is entirely opt-in via `traceLifecycle = 'stream'` and therefore does not introduce breaking changes to existing users.
 - The default tracing behavior (transaction-based) MUST remain unchanged until Span-First becomes the default in a future major release.
 - Release notes and user facing documentation SHOULD clearly describe:
   - the availability of Span-First behind the opt-in flag
-  - any known limitations
+  - any known limitations

develop-docs/sdk/telemetry/spans/span-api.mdx

@@ -195,3 +195,46 @@ with sentry_sdk.start_span(name="checkout", attributes={"user.id": "123"}) as ch
     log_order()
     span.end()
 ```
+
+```Dart {tabTitle:Dart}
+final checkoutSpan = Sentry.startSpan(
+  'on-checkout-click',
+  attributes: {
+    'user.id': SentryAttribute.string('123'),
+  },
+);
+
+final validationSpan = Sentry.startSpan('validate-shopping-cart');
+
+startFormValidation().then((result) {
+  validationSpan.setAttribute('valid-form-data', SentryAttribute.bool(result.success));
+  validationSpan.end();
+});
+
+final processSpan = Sentry.startSpan(
+  'process-order',
+  parentSpan: checkoutSpan,
+);
+
+processOrder().then((result) {
+  processSpan.setAttribute('order-processed', SentryAttribute.bool(result.success));
+  processSpan.end();
+}).catchError((error) {
+  processSpan.status = SpanV2Status.error;
+  processSpan.setAttribute('order-processed', SentryAttribute.string('error'));
+  processSpan.end();
+});
+
+final unrelatedSpan = Sentry.startSpan(
+  'log-order',
+  parentSpan: null,
+);
+
+logOrder();
+
+unrelatedSpan.end();
+
+on('checkout-finished', (timestamp) {
+  checkoutSpan.end(endTimestamp: timestamp);
+});
+```