modules/find: param contains: Describe matching behavior (#80744)

lib/ansible/modules/find.py

@@ -56,6 +56,9 @@ options:
     contains:
         description:
             - A regular expression or pattern which should be matched against the file content.
+            - If I(read_whole_file) is C(true) it matches against the beginning of the line (uses
+              C(re.match())). If I(read_whole_file) is C(false), it searches anywhere for that pattern
+              (uses C(re.search())).
             - Works only when I(file_type) is C(file).
         type: str
     read_whole_file: