More read-only-allow-delete docs (#45320)

Adds to the `index.blocks.read_only_allow_delete` docs the information that
this block may be added or removed automatically.

Relates #42559

docs/reference/index-modules.asciidoc

@@ -166,8 +166,10 @@ specific index module:
     allow writes and metadata changes.
 
 `index.blocks.read_only_allow_delete`::
-    Identical to `index.blocks.read_only` but allows deleting the index to free
-     up resources.
+
+    Similar to `index.blocks.read_only` but also allows deleting the index to
+    free up resources. The <<disk-allocator,disk-based shard allocator>> may
+    add and remove this block automatically.
 
 `index.blocks.read`::