Makefile: $(MAKE) check-docs

Makefile: $(MAKE) check-docs

This target lists undocumented commands, and/or whose document
is not referenced from the main git documentation.

For now, there are some exceptions I added primarily because I
lack the energy to document them myself:

 - merge backends (we should really document them)
 - ssh-push/ssh-pull (does anybody still use them?)
 - annotate and blame (maybe after one of them eats the other ;-)

Signed-off-by: Junio C Hamano <junkio@cox.net>