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>

Author: stefanbeller

Documentation/git-submodule.txt

@@ -154,37 +154,35 @@ Update the registered submodules to match what the superproject
 expects by cloning missing submodules and updating the working tree of
 the submodules. The "updating" can be done in several ways depending
 on command line options and the value of `submodule.<name>.update`
-configuration variable. Supported update procedures are:
+configuration variable. The command line option takes precedence over
+the configuration variable. if neither is given, a checkout is performed.
+update procedures supported both from the command line as well as setting
+`submodule.<name>.update`:
 
 	checkout;; the commit recorded in the superproject will be
-	    checked out in the submodule on a detached HEAD. This is
-	    done when `--checkout` option is given, or no option is
-	    given, and `submodule.<name>.update` is unset, or if it is
-	    set to 'checkout'.
+	    checked out in the submodule on a detached HEAD.
 +
 If `--force` is specified, the submodule will be checked out (using
 `git checkout --force` if appropriate), even if the commit specified
 in the index of the containing repository already matches the commit
 checked out in the submodule.
 
 	rebase;; the current branch of the submodule will be rebased
-	    onto the commit recorded in the superproject. This is done
-	    when `--rebase` option is given, or no option is given, and
-	    `submodule.<name>.update` is set to 'rebase'.
+	    onto the commit recorded in the superproject.
 
 	merge;; the commit recorded in the superproject will be merged
-	    into the current branch in the submodule. This is done
-	    when `--merge` option is given, or no option is given, and
-	    `submodule.<name>.update` is set to 'merge'.
+	    into the current branch in the submodule.
+
+The following procedures are only available via the `submodule.<name>.update`
+configuration variable:
 
 	custom command;; arbitrary shell command that takes a single
 	    argument (the sha1 of the commit recorded in the
-	    superproject) is executed. This is done when no option is
-	    given, and `submodule.<name>.update` has the form of
-	    '!command'.
+	    superproject) is executed. When `submodule.<name>.update`
+	    is set to '!command', the remainder after the exclamation mark
+	    is the custom command.
 
-When no option is given and `submodule.<name>.update` is set to 'none',
-the submodule is not updated.
+	none;; the submodule is not updated.
 
 If the submodule is not yet initialized, and you just want to use the
 setting as stored in .gitmodules, you can automatically initialize the