#775964: skip YP/NIS entries instead of failing the test

Also includes doc updates mentioning that these entries may not
be retrievable via getgrnam and getgrgid.

Patch by Bobby Impollonia.
diff --git a/Doc/library/grp.rst b/Doc/library/grp.rst
index 57f160a..8882140 100644
--- a/Doc/library/grp.rst
+++ b/Doc/library/grp.rst
@@ -30,7 +30,9 @@
 The gid is an integer, name and password are strings, and the member list is a
 list of strings. (Note that most users are not explicitly listed as members of
 the group they are in according to the password database.  Check both databases
-to get complete membership information.)
+to get complete membership information.  Also note that a ``gr_name`` that
+starts with a ``+`` or ``-`` is likely to be a YP/NIS reference and may not be
+accessible via :func:`getgrnam` or :func:`getgrgid`.)
 
 It defines the following items: