git-config(1) should mention `git -c`

In addition to describing the `git config` command, git-config(1) also appears to be where the algorithm for determining the active configuration values is documented, in the FILES and ENVIRONMENT sections. (There is minimal documentation of these files and environment variables in git(1).)

However, `git -c name=value` is not documented in this manpage. So to understand the full set of ways to affect the configuration of a git command, you need to know to read both git(1) and git-config(1).

I'd like to add a reference to `git -c` to git-config(1). I would happily send a patch but I'm not sure of the best place to put it — maybe in the FILES section?

--dave