fetch/pull: Describe --recurse-submodule restrictions in the BUGS section

Using the --recurse-submodules option with fetch and pull might not always
fetch all the submodule commits the user expects, as this will only work
when the submodule is already checked out. Document that and warn that
this is expected to change in the future.

Signed-off-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-fetch.txt

@@ -76,6 +76,15 @@ The `pu` branch will be updated even if it is does not fast-forward,
 because it is prefixed with a plus sign; `tmp` will not be.
 
 
+BUGS
+----
+Using --recurse-submodules can only fetch new commits in already checked
+out submodules right now. When e.g. upstream added a new submodule in the
+just fetched commits of the superproject the submodule itself can not be
+fetched, making it impossible to check out that submodule later without
+having to do a fetch again. This is expected to be fixed in a future git
+version.
+
 SEE ALSO
 --------
 linkgit:git-pull[1]

Documentation/git-pull.txt

@@ -220,6 +220,15 @@ If you tried a pull which resulted in a complex conflicts and
 would want to start over, you can recover with 'git reset'.
 
 
+BUGS
+----
+Using --recurse-submodules can only fetch new commits in already checked
+out submodules right now. When e.g. upstream added a new submodule in the
+just fetched commits of the superproject the submodule itself can not be
+fetched, making it impossible to check out that submodule later without
+having to do a fetch again. This is expected to be fixed in a future git
+version.
+
 SEE ALSO
 --------
 linkgit:git-fetch[1], linkgit:git-merge[1], linkgit:git-config[1]