[PATCHv2] Documentation/config: clarify the meaning of submodule.<name>.update

[Stefan Beller <sbeller@google.com>]

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/4283F0B0-BC1C-4ED1-8126-7E512D84484B@gmail.com/
    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 <larsxschneider@gmail.com>
Signed-off-by: Stefan Beller <sbeller@google.com>
---
 Documentation/config.txt | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

Jonathan writes:
> You'll want to update Documentation/gitmodules.txt, too.

No. /grumpycat

It should already be fine, as I read it as if it is only relevant to 
"git submodule update" there, already:

  submodule.<name>.update::
	Defines the default update procedure for the named submodule,
	i.e. how the submodule is updated by "git submodule update"
	command in the superproject. This is only used by `git
	submodule init` to initialize the configuration variable of
	the same name. Allowed values here are 'checkout', 'rebase',
	'merge' or 'none'. See description of 'update' command in
	linkgit:git-submodule[1] for their meaning. Note that the
	'!command' form is intentionally ignored here for security
	reasons.
 

diff --git a/Documentation/config.txt b/Documentation/config.txt
index dc4e3f58a2..1ac0ae6adb 100644
--- a/Documentation/config.txt
+++ b/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
-- 
2.14.0.rc0.3.g6c2e499285