Highlight variable templates for known_identity

known_identity_v and has_known_identity_v appear in a synopsis block,
but the text preceding it recommends to use known_identity and
has_known_identity instead. We should highlight that these helper variable
templates exist when they are first introduced.

adoc/chapters/programming_interface.adoc

@@ -13640,9 +13640,11 @@ identity value automatically in order to avoid performance penalties.
 
 If an implementation can identify an identity value for a given combination of
 accumulator type and function object type, the value is defined as a member of
-the [code]#known_identity# trait class.
+the [code]#known_identity# trait class and can be retrieved using the
+[code]#known_identity_v# variable template.
 Whether this member value exists can be tested using the
-[code]#has_known_identity# trait class.
+[code]#has_known_identity# trait class or the [code]#has_known_identity_v#
+variable template.
 
 [source,,linenums]
 ----