bpo-42416: Use inspect.getdoc for IDLE calltips (GH-23416)

Inspect.getdoc(ob) sometimes gets docstrings when ob.__doc__ is None.
diff --git a/Lib/idlelib/NEWS.txt b/Lib/idlelib/NEWS.txt
index 3ece623..c9f28f6 100644
--- a/Lib/idlelib/NEWS.txt
+++ b/Lib/idlelib/NEWS.txt
@@ -3,6 +3,9 @@
 ======================================
 
 
+bpo-42416: Get docstrings for IDLE calltips more often
+by using inspect.getdoc.
+
 bpo-33987: Mostly finish using ttk widgets, mainly for editor,
 settings, and searches.  Some patches by Mark Roseman.