Documentation/config: clarify the meaning of submodule.<name>.update

With more commands (that potentially change a submodule) paying attention
to submodules as well as the recent discussion[1] on
submodule.<name>.update, let's spell out that submodule.<name>.update
is strictly to be used for configuring the "submodule update" command
and not to be obeyed by other commands.

These other commands usually have a strict meaning of what they should
do (i.e. checkout, reset, rebase, merge) as well as have their name
overlapping with the modes possible for submodule.<name>.update.

[1] https://public-inbox.org/git/[email protected]/
    submodule.<name>.update was set to "none", triggering unexpected
    behavior as the submodule was thought to never be touched.
    However a newer version of Git taught 'git pull --rebase' to also
    populate and rebase submodules if they were active.
    The newer options such as submodule.active and command specific
    flags would not have triggered unexpected behavior.

Reported-by: Lars Schneider <[email protected]>
Signed-off-by: Stefan Beller <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: stefanbeller

Documentation/config.txt

@@ -3085,10 +3085,14 @@ submodule.<name>.url::
 	See linkgit:git-submodule[1] and linkgit:gitmodules[5] for details.
 
 submodule.<name>.update::
-	The default update procedure for a submodule. This variable
-	is populated by `git submodule init` from the
-	linkgit:gitmodules[5] file. See description of 'update'
-	command in linkgit:git-submodule[1].
+	The method by which a submodule is updated by 'git submodule update',
+	which is the only affected command, others such as
+	'git checkout --recurse-submodules' are unaffected. It exists for
+	historical reasons, when 'git submodule' was the only command to
+	interact with submodules; settings like `submodule.active`
+	and `pull.rebase` are more specific. It is populated by
+	`git submodule init` from the linkgit:gitmodules[5] file.
+	See description of 'update' command in linkgit:git-submodule[1].
 
 submodule.<name>.branch::
 	The remote branch name for a submodule, used by `git submodule