Flesh out bisect help text

hgext/hbisect.py

@@ -252,8 +252,21 b' def test(ui, repo, rev):'
     return 0
 
 def bisect_run(ui, repo, cmd=None, *args):
-    """bisect extension: dichotomic search in the DAG of changesets
-for subcommands see "hg bisect help\"
+    """Dichotomic search in the DAG of changesets
+
+This extension helps to find changesets which cause problems.
+To use, mark the earliest changeset you know introduces the problem
+as bad, then mark the latest changeset which is free from the problem
+as good. Bisect will update your working directory to a revision for
+testing. Once you have performed tests, mark the working directory
+as bad or good and bisect will either update to another candidate
+changeset or announce that it has found the bad revision.
+
+Note: bisect expects bad revisions to be descendants of good revisions.
+If you are looking for the point at which a problem was fixed, then make
+the problem-free state "bad" and the problematic state "good."
+
+For subcommands see "hg bisect help\"
     """
     def help_(cmd=None, *args):
         """show help for a given bisect subcommand or all subcommands"""