bisect: add example for limiting bisection to specified directories

The bisect command does not have an option to limit itself only to
subdirectories, but it's possible to use revsets for the --skip option
for the same effect. Given the relative obscurity of revsets, it helps
to have this as another example for bisect.

mercurial/commands.py

@@ -549,6 +549,10 b' def bisect(ui, repo, rev=None, extra=Non'
           hg bisect --skip
           hg bisect --skip 23
 
+      - skip all revisions that do not touch directories ``foo`` or ``bar``
+
+          hg bisect --skip '!( file("path:foo") & file("path:bar") )'
+
       - forget the current bisection::
 
           hg bisect --reset