doc: make it easier to find custom command information

doc: make it easier to find custom command information

Git supports creating additional commands through aliases, and through
placement of executables with a "git-" prefix in the PATH.

This information was not easy enough to find - users will look for this
information around the command description, but the documentation
exists in other locations.

Update the "GIT COMMANDS" section to reference the relevant sections,
making it easier for to find this information.

Signed-off-by: Omri Sarig <omri.sarig13@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>