git-clone -s: document problems with git gc --prune

There is a scenario when using git clone -s and git gc --prune togother is
dangerous. Document this.
Signed-off-by: NMiklos Vajna <vmiklos@frugalware.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

Documentation/git-clone.txt

@@ -62,6 +62,14 @@ OPTIONS
 	.git/objects/info/alternates to share the objects
 	with the source repository.  The resulting repository
 	starts out without any object of its own.
+	*NOTE*: this is a possibly dangerous operation; do *not* use
+	it unless you understand what it does. If you clone your
+	repository using this option, then delete branches in the
+	source repository and then run linkgit:git-gc[1] using the
+	'--prune' option in the source repository, it may remove
+	objects which are referenced by the cloned repository.
+
+
 
 --reference <repository>::
 	If the reference repository is on the local machine