Add CHANGES and release note for [GL #4621]

(cherry picked from commit 79ee08c337)

CHANGES

@@ -1,3 +1,7 @@
+6356.	[bug]		Create the pruning task in the dns_cache_flush(), so
+			the cache pruning still works after the flush.
+			[GL #4621]
+
 6353.	[bug]		Improve the TTL-based cleaning by removing the expired
 			headers from the heap, so they don't block the next
 			cleaning round and clean more than a single item for

doc/notes/notes-current.rst

@@ -39,6 +39,11 @@ Bug Fixes
   significantly more quickly than before, until the configured
   ``max-cache-size`` limit was reached. This has been fixed. :gl:`#4596`
 
+- Using :option:`rndc flush` inadvertently caused cache cleaning to
+  become less effective. This could ultimately lead to the configured
+  ``max-cache-size`` limit being exceeded and has now been fixed.
+  :gl:`#4621`
+
 Known Issues
 ~~~~~~~~~~~~