doc/SubmittingPatches: correct subject guidance

doc/SubmittingPatches: correct subject guidance

The examples and common practice for adding markers such as "RFC" or
"v2" to the subject of patch emails is to have them within the same
brackets as the "PATCH" text, not after the closing bracket.  Further,
the practice of `git format-patch` and the like, as well as what appears
to be the more common pratice on the mailing list, is to use "[RFC
PATCH]", not "[PATCH/RFC]".

Update the SubmittingPatches article to match and to reference the
`format-patch` helper arguments, and also make some minor text
clarifications in the area.

Signed-off-by: Adam Dinwoodie <adam@dinwoodie.org>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>