check-docs: update non-command documentation list

check-docs: update non-command documentation list

The check-docs target looks at Documentation/git*txt and
complains if any entry does not have a matching command.
Therefore we need to explicitly ignore any entries which are
not meant to describe a command (like gitattributes.txt).
This list has grown stale over time, so let's bring it up to
date.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>