Clarification on usefulness of %h<a>:!exists

After having been caught by this a few times while adapting the test-suite.

Author: lizmat

S02-bits.pod

@@ -2528,7 +2528,9 @@ similar semantics:
 
 although with different optimization options for the compiler.
 
-You may use :!exists to test for non-existence.
+You may use :!exists to test for non-existence.  This is specifically handy
+because of precedence rules making C<!%hash<a>:exists> apply the :exists to the
+prefix C<!>.  C<%hash<a>:!exists> does not have that problem.
 
 =head2 Combining subscript adverbs