doc: clarify "do not capitalize the first word" rule

doc: clarify "do not capitalize the first word" rule

The same "do not capitalize the first word" rule is applied to both
our patch titles and error messages, but the existing description
was fuzzy in two aspects.

 * For error messages, it was not said that this was only about the
   first word that begins the sentence.

 * For both, it was not clear when a capital letter there was not an
   error.  We avoid capitalizing the first word when the only reason
   you would capitalize it is because it happens to be the first
   word in the sentence.  If a proper noun, which is usually spelled
   in capital letters, happens to come at the beginning of the
   sentence, it should be kept in capital letters.

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