Documentation of the xmethod support in GDB Python API.

	* NEWS (Python Scripting): Add entry about the new xmethods
	feature.

	doc/
	*  python.texi (Xmethods In Python, XMethod API)
	(Writing an Xmethod): New nodes.
	(Python API): New menu entries "Xmethods In Python",
	"Xmethod API", "Writing an Xmethod".
gdb/doc/python.texi: Fix reference to "Progspaces In Python".

A recent change broke the documentation build due to a think-o
in a reference.  Fixed thusly.

gdb/doc/ChangeLog:

        * python.texi (Xmethod API): Fix reference to "Progspaces In
        Python".

Tested by rebuilding all documentation formats.