gdb: Make python display_hint None handling defined behaviour

The documentation say that the display_hint method must return a
string to serve as a display hint, and then goes on to list some
acceptable strings.

However, if we don't supply the display_hint method then we get a
default display style behaviour and there's currently no way (in the
python api) to force this default behaviour.

The guile api allows #f to be used in order to force the default
display style behaviour, and this is documented.

Currently, using None in the python api also forces the default
display behaviour.

This commit extends the documentation to make returning None from the
display_hint method an official mechanism by which the user can get
the default display style.

I've extended one of the existing tests to cover this case.

gdb/doc/ChangeLog:

	* python.texi (Pretty Printing API): Document use of None for the
	display_hint.

gdb/testsuite/ChangeLog:

	* gdb.python/py-prettyprint.c (struct container) <is_map_p>: New
	field.
	(make_container): Initialise new field.
	* gdb.python/py-prettyprint.exp: Add new tests.
	* gdb.python/py-prettyprint.py (class ContainerPrinter)
	<display_hint>: New method.

gdb/testsuite/gdb.python/py-prettyprint.py

@@ -56,6 +56,12 @@ class ContainerPrinter (object):
     def children(self):
         return _iterator(self.val['elements'], self.val['len'])
 
+    def display_hint (self):
+        if (self.val['is_map_p']):
+            return 'map'
+        else:
+            return None
+
 # Treats a container as array.
 class ArrayPrinter (object):
     def __init__(self, val):