[PATCH v4] docs: highlight that .gitmodules does not support !command

[pvutov@imap.cc]

From: Petar Vutov <pvutov@imap.cc>

Bugfix for fc01a5d2 (submodule update documentation: don't repeat
ourselves, 2016-12-27).

The `custom command` and `none` entries are described as sharing the
same limitations, but one is allowed in .gitmodules and the other is
not. Instead, highlight the limitation of `custom command` only.

Signed-off-by: Petar Vutov <pvutov@imap.cc>
---

Changes from v1:
Don't delete the documentation for `!command`. Instead, highlight
the differences in the limitations of `none` and `!command`.

Changes from v2:
Improve phrasing.
Add the bugfix blurb in the commit message.

Changes from v3:
Keep the description of `none` unchanged and just move it up.

 Documentation/git-submodule.txt | 8 +++-----
 1 file changed, 3 insertions(+), 5 deletions(-)

diff --git a/Documentation/git-submodule.txt b/Documentation/git-submodule.txt
index 4d3ab6b9f9..b3303afb8f 100644
--- a/Documentation/git-submodule.txt
+++ b/Documentation/git-submodule.txt
@@ -160,16 +160,14 @@ checked out in the submodule.
 	merge;; the commit recorded in the superproject will be merged
 	    into the current branch in the submodule.
 
-The following 'update' procedures are only available via the
-`submodule.<name>.update` configuration variable:
+	none;; the submodule is not updated.
 
 	custom command;; arbitrary shell command that takes a single
 	    argument (the sha1 of the commit recorded in the
 	    superproject) is executed. When `submodule.<name>.update`
 	    is set to '!command', the remainder after the exclamation mark
-	    is the custom command.
-
-	none;; the submodule is not updated.
+	    is the custom command. Note that this mechanism
+	    cannot be used in the .gitmodules file.
 
 If the submodule is not yet initialized, and you just want to use the
 setting as stored in `.gitmodules`, you can automatically initialize the
-- 
2.41.0