submodule: Fix documentation of update subcommand

submodule: Fix documentation of update subcommand

The documentation of 'git submodule update' has several problems:

1) It says that submodule.$name.update can be overridden by --checkout
   only if its value is `none`. This is not true, because both
   implementation and documentation of --checkout specifies that the
   override applies to all possible values.

2) The documentation of submodule.$name.update key is scattered across
   three places, which is confusing.

3) The documentation of submodule.$name.update in gitmodules.txt is
   incorrect, because the code always uses the value from .git/config
   and never from .gitmodules.

This patch fixes all three problems. Now, submodule.$name.update is
fully documented in config.txt and the other files just refer to it.
This is based on discussion between myself, Junio C Hamano and Jens
Lehmann.

Signed-off-by: Michal Sojka <sojkam1@fel.cvut.cz>