git-clone.txt: Adjust note to --shared for new pruning behavior of git-gc

Since git-gc now always calls prune, even with --auto, unreferenced objects
may be removed by more operations than just git-gc. This is important for
clones created using --shared or --reference.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Dmitry Potapov <dpotapov@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-clone.txt

@@ -65,10 +65,13 @@ OPTIONS
 +
 *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
+repository using this option and then delete branches (or use any
-source repository and then run linkgit:git-gc[1] using the
+other git command that makes any existing commit unreferenced) in the
-'--prune' option in the source repository, it may remove
+source repository, some objects may become unreferenced (or dangling).
-objects which are referenced by the cloned repository.
+These objects may be removed by normal git operations (such as git-commit[1])
+which automatically call git-gc[1]. If these objects are removed and
+were referenced by the cloned repository, then the cloned repository
+will become corrupt.
 
 
 
@@ -79,6 +82,8 @@ objects which are referenced by the cloned repository.
 	an already existing repository as an alternate will
 	require fewer objects to be copied from the repository
 	being cloned, reducing network and local storage costs.
++
+*NOTE*: see NOTE to --shared option.
 
 --quiet::
 -q::