doc: be more precise on (fetch|push).recurseSubmodules

doc: be more precise on (fetch|push).recurseSubmodules

The default value also depends on the value of submodule.recurse.
Use this opportunity to correct some grammar mistakes in
Documentation/config/fetch.txt signaled by Robert P. J. Day.

Also mention `fetch.recurseSubmodules` in fetch-options.txt. In
git-push.txt, `push.recurseSubmodules` is implicitly mentioned (by
explaining how to disable it), so no need to add it there.

Lastly add a link to `git-fetch` in `git-pull.txt` to explain the
meaning of `--recurse-submodules` there.

Signed-off-by: Damien Robert <damien.olivier.robert+git@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>