fix depends_on :formula backend

closes #6158
restore `pkg` to cask_language_deltas.md

was deleted due to merge error in #6138
functionality and docs for zap stanza

* `zap` was previously supported as a noop for forward-compatibility
* Also adds restrictions against relative paths in `uninstall :files`
* Also documents safety conventions for `uninstall :files`
zap/uninstall :files in authoxy.rb
zap/uninstall :files in baiduinput.rb
zap/uninstall :files in brytonbridge.rb
zap/uninstall :files in carlson-minot.rb
zap/uninstall :files in cocoaspell.rb
zap/uninstall :files in craftstudio.rb
zap/uninstall :files in fender-fuse.rb
zap/uninstall :files in flash.rb
zap/uninstall :files in gpgtools.rb
zap/uninstall :files in hall.rb
zap/uninstall :files in injection.rb
zap/uninstall :files in mailfollowup.rb
zap/uninstall :files in netbeans-cpp.rb
zap/uninstall :files in netbeans-php.rb
zap/uninstall :files in netbeans.rb
zap/uninstall :files in nomachine.rb
zap/uninstall :files in octave.rb