docs: clarify that passing --depth to git-clone implies --single-branch

It is confusing to document how --depth behaves as part of the
--single-branch docs. Better move that part to the --depth docs, saying
that it implies --single-branch by default.

Signed-off-by: Sebastian Schuberth <sschuberth@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clone.txt

@@ -178,15 +178,14 @@ objects from the source repository into a pack in the cloned repository.
 
 --depth <depth>::
 	Create a 'shallow' clone with a history truncated to the
-	specified number of revisions.
+	specified number of revisions. Implies `--single-branch` unless
+	`--no-single-branch` is given to fetch the histories near the
+	tips of all branches.
 
 --[no-]single-branch::
 	Clone only the history leading to the tip of a single branch,
 	either specified by the `--branch` option or the primary
-	branch remote's `HEAD` points at. When creating a shallow
-	clone with the `--depth` option, this is the default, unless
-	`--no-single-branch` is given to fetch the histories near the
-	tips of all branches.
+	branch remote's `HEAD` points at.
 	Further fetches into the resulting repository will only update the
 	remote-tracking branch for the branch this option was used for the
 	initial cloning.  If the HEAD at the remote did not point at any