Add some documentation about the "user" key in the "regex" rights module

radicale/rights/regex.py

@@ -29,6 +29,9 @@ is matched against the "collection" key. You can use Python's ConfigParser
 interpolation values %(login)s and %(path)s. You can also get groups from the
 user regex in the collection with {0}, {1}, etc.
 
+For example, for the "user" key, ".+" means "authenticated user" and ".*"
+means "anybody" (including anonymous users).
+
 Section names are only used for naming the rule.
 
 Leading or ending slashes are trimmed from collection's path.