Add breadcrumbs for assertEventQueueAfterClose and closeInternal removals, #1284

Upstream 2cfcdcc rewrote close(boolean) to delegate to shutdown(boolean)
but kept assertEventQueueAfterClose() and closeInternal(boolean, boolean)
as private dead code. The original #1284 port (7ec8790) dropped both,
which matches trunk LUCENE-4246's eventual cleanup (8559eaf, Lucene 5.0)
but diverges from the conservative branch_4x state at 2cfcdcc.

Adds two LUCENENET breadcrumbs around ShouldClose() so a future porter
diffing against 2cfcdcc can find the removal rationale at the position
where each method used to live:

- Full note before ShouldClose() covering both, since
  assertEventQueueAfterClose() sat there in upstream.
- One-liner after ShouldClose() pointing back to the full note, since
  closeInternal() sat immediately after.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: paulirwin

src/Lucene.Net/Index/IndexWriter.cs

@@ -1227,6 +1227,14 @@ private void Shutdown(bool waitForMerges)
             }
         }
 
+        // LUCENENET: upstream 2cfcdcc had private assertEventQueueAfterClose() between
+        // close(boolean) and shouldClose() (and closeInternal(boolean, boolean) immediately
+        // after shouldClose()). The LUCENE-5871 rewrite of close(boolean) stranded both as
+        // dead code: shutdown(boolean) replaced their only callers, and neither is referenced
+        // anywhere else in the file at 2cfcdcc. Trunk LUCENE-4246 (commit 8559eaf, Lucene 5.0)
+        // removed both alongside the rest of the close(boolean) cleanup; we follow that 5.0
+        // end state here rather than carrying the dead branch_4x leftovers.
+
         /// <summary>
         /// Returns <c>true</c> if this thread should attempt to close, or
         /// false if IndexWriter is now closed; else, waits until
@@ -1266,6 +1274,9 @@ private bool ShouldClose()
             }
         }
 
+        // LUCENENET: upstream 2cfcdcc had private closeInternal(boolean waitForMerges,
+        // boolean doFlush) here. See the note above ShouldClose() for why it is dropped.
+
         /// <summary>
         /// Gets the <see cref="Store.Directory"/> used by this index. </summary>
         public virtual Directory Directory => directory;