[SPARK-56798][SQL][DOCS] Clarify streaming CDC emission timing and netChanges scope

sql/catalyst/src/main/java/org/apache/spark/sql/connector/catalog/Changelog.java

@@ -71,10 +71,21 @@
  * </ul>
  * <p>
  * Streaming reads support carry-over removal, update detection, and net change
- * computation. Net change collapses are kept in the state store keyed by row identity;
- * row identities only touched in the latest observed commit are held back until either a
- * later commit (with strictly greater `_commit_timestamp`) advances the global watermark
- * past them, or the source terminates.
+ * computation. Two streaming-specific behaviors to be aware of:
+ * <ul>
+ *   <li><b>Output is buffered until the watermark advances past the commit.</b>
+ *       When a micro-batch ingests a commit, that commit's output rows are
+ *       buffered in state and not emitted in the same batch. They are emitted
+ *       by a later micro-batch -- whichever one advances the watermark past
+ *       the commit's {@code _commit_timestamp}. The last commit's output is
+ *       emitted when the source terminates.</li>
+ *   <li><b>netChanges only merges changes that are buffered together.</b>
+ *       When each row identity appears in at most one commit within any
+ *       buffered window, the streaming output is the same as
+ *       {@code computeUpdates}. Cross-commit merging only happens when
+ *       several commits touch the same row before the earliest one's output
+ *       has been released. For full-range collapse, use a batch read.</li>
+ * </ul>
  * <p>
  * <b>Pushdown contract.</b> When any post-processing pass applies (carry-over
  * removal, update detection, or netChanges), Spark only pushes predicates