Merge r1631983 from trunk:

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

Submitted by: rjung
Reviewed by: jim, ylavic


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1634948 13f79535-47bb-0310-9956-ffa450edef68