docs(extend-app-start): Document IAppStartExtender method contracts

Add behavior-focused javadoc (call site, idempotency, no-op conditions) to
extendAppStart/finishAppStart rather than restating the method names.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: buenaflor

sentry/src/main/java/io/sentry/IAppStartExtender.java

@@ -12,8 +12,17 @@
 @ApiStatus.Internal
 public interface IAppStartExtender {
 
+  /**
+   * Begins extending the app start. Intended to be called from {@code Application.onCreate} right
+   * after SDK init. No-ops if the app start already finished, none is in progress, or it was
+   * already extended (first call wins).
+   */
   void extendAppStart();
 
+  /**
+   * Finishes the extended app start, allowing the app start transaction to complete. No-ops if the
+   * app start was not extended or this was already called.
+   */
   void finishAppStart();
 
   /**