doc/SubmittingPatches: clarify the casing convention for "area: change..."

Amend the section which describes how the first line of the subject
should look like to say that the ":" in "area: " shouldn't be treated
like a full stop for the purposes of letter casing.

Change the two subject examples to make this new paragraph clearer,
i.e. "unstar" is not a common word, and "git-cherry-pick.txt" is a
much longer string than "githooks.txt". Pick two recent commits from
git.git that fit better for the description.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: avar

Documentation/SubmittingPatches

@@ -98,12 +98,17 @@ should skip the full stop.  It is also conventional in most cases to
 prefix the first line with "area: " where the area is a filename or
 identifier for the general area of the code being modified, e.g.
 
-  . archive: ustar header checksum is computed unsigned
-  . git-cherry-pick.txt: clarify the use of revision range notation
+  . doc: clarify distinction between sign-off and pgp-signing
+  . githooks.txt: improve the intro section
 
 If in doubt which identifier to use, run "git log --no-merges" on the
 files you are modifying to see the current conventions.
 
+It's customary to start the remainder of the first line after "area: "
+with a lower-case letter. E.g. "doc: clarify...", not "doc:
+Clarify...", or "githooks.txt: improve...", not "githooks.txt:
+Improve...".
+
 The body should provide a meaningful commit message, which:
 
   . explains the problem the change tries to solve, iow, what is wrong