docs: drop antique comment from Makefile
docs: drop antique comment from Makefile

This comment warns about a bug in asciidoc 6, and points to
a patch from 2005. Since we don't even support versions of
asciidoc that old, we can safely get rid of the warning.

Signed-off-by: Jeff King <696e3fbcf235d40b6ea1ebd61c87cbea79d444e7@peff.net>
Signed-off-by: Junio C Hamano <a6723cc3f76163bf7adb636a73ac3b0ceb3e6b9b@pobox.com>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git