Mention in Naming Conventions that is_ and trailing ? should not be used together (#14679)

Author: eksperimental

lib/elixir/pages/references/naming-conventions.md

@@ -109,6 +109,8 @@ Examples: `Integer.is_even/1`, `is_list/1`
 
 These functions and macros follow the Erlang convention of an `is_` prefix, instead of a trailing question mark, precisely to indicate that they are allowed in guard clauses.
 
+A trailing question mark should not be used in combination with the `is_` prefix.
+
 Note that type checks that are not valid in guard clauses do not follow this convention. For example: `Keyword.keyword?/1`.
 
 ## Special names