submodule update documentation: don't repeat ourselves

submodule update documentation: don't repeat ourselves

The documentation for the `git submodule update` command, repeats itself
for each update option, "This is done when <option> is given, or no
option is given and `submodule.<name>.update` is set to <string>.

Avoid these repetitive clauses by stating the command line options take
precedence over configured options.

Also add 'none' to the list of options instead of mentioning it in the
following running text and split the list into two parts, one that is
accessible via the command line and one that is only reachable via the
configuration variables.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>