"git prune" is safe

"git prune" is safe in case of concurrent accesses to a repository
but using it in such a case is not recommended.

Signed-off-by: Thomas Ackermann <th.acker@arcor.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/user-manual.txt

@@ -3299,17 +3299,11 @@ state, you can just prune all unreachable objects:
 $ git prune
 ------------------------------------------------
 
-and they'll be gone. But you should only run `git prune` on a quiescent
+and they'll be gone. (You should only run `git prune` on a quiescent
 repository--it's kind of like doing a filesystem fsck recovery: you
 don't want to do that while the filesystem is mounted.
-
-(The same is true of `git fsck` itself, btw, but since
-`git fsck` never actually *changes* the repository, it just reports
-on what it found, `git fsck` itself is never 'dangerous' to run.
-Running it while somebody is actually changing the repository can cause
-confusing and scary messages, but it won't actually do anything bad. In
-contrast, running `git prune` while somebody is actively changing the
-repository is a *BAD* idea).
+`git prune` is designed not to cause any harm in such cases of concurrent
+accesses to a repository but you might receive confusing or scary messages.)
 
 [[recovering-from-repository-corruption]]
 Recovering from repository corruption