#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/Modules/grpmodule.c b/Modules/grpmodule.c
index 97a3783..7dfda28 100644
--- a/Modules/grpmodule.c
+++ b/Modules/grpmodule.c
@@ -159,7 +159,9 @@
 name is not valid, raise KeyError."},
     {"getgrall",        grp_getgrall,   METH_NOARGS,
      "getgrall() -> list of tuples\n\
-Return a list of all available group entries, in arbitrary order."},
+Return a list of all available group entries, in arbitrary order.\n\
+An entry whose name starts with '+' or '-' represents an instruction\n\
+to use YP/NIS and may not be accessible via getgrnam or getgrgid."},
     {NULL,              NULL}           /* sentinel */
 };