Merge branch 'dg/document-git-c-in-git-config-doc'

The "git -c var[=val] cmd" facility to append a configuration variable definition at the end of the search order was described in git(1) manual page, but not in git-config(1), which was more likely place for people to look for when they ask "can I make a one-shot override, and if so how?" * dg/document-git-c-in-git-config-doc: doc: mention `git -c` in git-config(1)