Merge r1628919, r1628950 from trunk:

mod_substitute: Make maximum line length configurable.


Add docs for new directive SubstituteMaxLineLength
in mod_substitute.

Submitted by: rjung
Reviewed/backported by: jim


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1634523 13f79535-47bb-0310-9956-ffa450edef68