documentation: add documentation for 'git version'

While 'git version' is probably the least complex git command,
it is a non-experimental user-facing builtin command. As such
it should have a help page.

Both `git help` and `git version` can be called as options
(`--help`/`--version`) that internally get converted to the
corresponding command. Add a small paragraph to
Documentation/git.txt describing how these two options
interact with each other and link to this help page for the
sub-options that `--version` can take. Well, currently there
is only one sub-option, but that could potentially increase
in future versions of Git.

Signed-off-by: Matthias Aßhauer <mha1993@live.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
