share: be more explicit about the dangers of rollback.

hgext/share.py

@@ -14,8 +14,15 b' def share(ui, source, dest=None, noupdat'
     Initialize a new repository and working directory that shares its
     history with another repository.
 
-    NOTE: actions that change history such as rollback or moving the
-    source may confuse sharers.
+    NOTE: using rollback or extensions that destroy/modify history
+    (mq, rebase, etc.) can cause considerable confusion with shared
+    clones. In particular, if two shared clones are both updated to
+    the same changeset, and one of them destroys that changeset with
+    rollback, the other clone will suddenly stop working: all
+    operations will fail with "abort: working directory has unknown
+    parent". The only known workaround is to use debugsetparents on
+    the broken clone to reset it to a changeset that still exists
+    (e.g. tip).
     """
 
     return hg.share(ui, source, dest, not noupdate)