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

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

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)