Fix mod_headers documentation:

- add expr= to syntax rule
- add optional replacement argument to Header syntax
- add "edit*" command to Header syntax
- add "edit*" explanation to Header directive

Backport of r1371903 from trunk.


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1371911 13f79535-47bb-0310-9956-ffa450edef68