mod_substitute: No real functional change, but:

- define default for maximum line length
  indepenent of MAX_STRING_LEN
- Clarify line length check with a comment
- add supported 'b' suffix to directive
  description


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1631983 13f79535-47bb-0310-9956-ffa450edef68