Add CHANGES and release note for [GL #4621]

CHANGES

@@ -1,4 +1,6 @@
-6356.	[placeholder]
+6356.	[bug]		Attach the loop also in the dns_cache_flush(), so
+			the cache pruning still works after the flush.
+			[GL #4621]
 
 6355.	[func]		The red-black tree data structure underlying the
 			RBTDB has been replaced with QP-tries.  This is

doc/notes/notes-current.rst

@@ -92,6 +92,11 @@ Bug Fixes
   :any:`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
+  :any:`max-cache-size` limit being exceeded and has now been fixed.
+  :gl:`#4621`
+
 Known Issues
 ~~~~~~~~~~~~