enable doctest plugin by default, add a --doctest-glob option and some documentation, regen plugin docs.

--HG--
branch : trunk

doc/test/plugin/hookspec.txt

@@ -32,6 +32,7 @@ hook specification sourcecode
     
     def pytest_collect_directory(path, parent):
         """ return Collection node or None for the given path. """
+    pytest_collect_directory.firstresult = True
     
     def pytest_collect_file(path, parent):
         """ return Collection node or None for the given path. """