doc: highlight that .gitmodules does not support !command

doc: highlight that .gitmodules does not support !command

Bugfix for fc01a5d2 (submodule update documentation: don't repeat
ourselves, 2016-12-27).

The `custom command` and `none` options are described as sharing the
same limitations, but one is allowed in .gitmodules and the other is
not.

Rewrite the description for custom commands to be more precise,
and make it easier for readers to notice that custom commands cannot
be used in the .gitmodules file.

Signed-off-by: Petar Vutov <pvutov@imap.cc>
Signed-off-by: Junio C Hamano <gitster@pobox.com>