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


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1371903 13f79535-47bb-0310-9956-ffa450edef68