docs(#1445): note at-most-once GC concurrency semantics

T3.1 deferral holding pattern for 2.3: document current behavior so
quarantine-based serialization can land in 2.4 without retracting
stronger claims.

Author: dimitri-yatsenko

src/how-to/garbage-collection.md

@@ -16,6 +16,14 @@ remain. This is by design:
 
 Run garbage collection periodically to reclaim storage space.
 
+!!! note "Concurrency semantics"
+    Garbage collection is at-most-once: it scans references, then deletes
+    unreferenced objects. An object inserted between the scan and the delete
+    may be briefly orphaned. A future release will add quarantine-based
+    serialization to close this window — until then, prefer running GC during
+    quiet periods, and treat any single GC pass as best-effort rather than
+    transactionally safe.
+
 ## Basic Usage
 
 ```python