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

Makefile

@@ -2853,6 +2853,9 @@ check-docs::
 		documented,git-bisect-lk2009 | \
 		documented,git-remote-helpers | \
 		documented,gitworkflows | \
+		documented,gitcredentials | \
+		documented,gitnamespaces | \
+		documented,gitweb.conf | \
 		sentinel,not,matching,is,ok ) continue ;; \
 		esac; \
 		case " $(ALL_PROGRAMS) $(SCRIPT_LIB) $(BUILT_INS) git gitk gitweb " in \