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>

Documentation/SubmittingPatches

@@ -184,14 +184,15 @@ lose tabs that way if you are not careful.
 
 It is a common convention to prefix your subject line with
 [PATCH].  This lets people easily distinguish patches from other
-e-mail discussions.  Use of additional markers after PATCH and
-the closing bracket to mark the nature of the patch is also
-encouraged.  E.g. [PATCH/RFC] is often used when the patch is
-not ready to be applied but it is for discussion, [PATCH v2],
-[PATCH v3] etc. are often seen when you are sending an update to
-what you have previously sent.
-
-"git format-patch" command follows the best current practice to
+e-mail discussions.  Use of markers in addition to PATCH within
+the brackets to describe the nature of the patch is also
+encouraged.  E.g. [RFC PATCH] (where RFC stands for "request for
+comments") is often used to indicate a patch needs further
+discussion before being accepted, [PATCH v2], [PATCH v3] etc.
+are often seen when you are sending an update to what you have
+previously sent.
+
+The "git format-patch" command follows the best current practice to
 format the body of an e-mail message.  At the beginning of the
 patch should come your commit message, ending with the
 Signed-off-by: lines, and a line that consists of three dashes,
@@ -199,6 +200,10 @@ followed by the diffstat information and the patch itself.  If
 you are forwarding a patch from somebody else, optionally, at
 the beginning of the e-mail message just before the commit
 message starts, you can put a "From: " line to name that person.
+To change the default "[PATCH]" in the subject to "[<text>]", use
+`git format-patch --subject-prefix=<text>`.  As a shortcut, you
+can use `--rfc` instead of `--subject-prefix="RFC PATCH"`, or
+`-v <n>` instead of `--subject-prefix="PATCH v<n>"`.
 
 You often want to add additional explanation about the patch,
 other than the commit message itself.  Place such "cover letter"