SubmittingPatches: use usual capitalization in the log message body

Update the description of the summary section to clarify that the
"do not capitalize" rule applies only the word after the "<area>:"
prefix of the title and nowhere else.  This hopefully will prevent
folks from writing their proposed log message in all lowercase.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/SubmittingPatches

@@ -153,7 +153,9 @@ files you are modifying to see the current conventions.
 
 [[summary-section]]
 The title sentence after the "area:" prefix omits the full stop at the
-end, and its first word is not capitalized unless there is a reason to
+end, and its first word is not capitalized (the omission
+of capitalization applies only to the word after the "area:"
+prefix of the title) unless there is a reason to
 capitalize it other than because it is the first word in the sentence.
 E.g. "doc: clarify...", not "doc: Clarify...", or "githooks.txt:
 improve...", not "githooks.txt: Improve...".  But "refs: HEAD is also