Use @defvar to document gdb.pretty_printers

Use @defvar to document gdb.pretty_printers

While referencing the manual, I noticed that gdb.pretty_printers
wasn't documented using @defvar.  This made it more difficult to find
in the info pages.  This patch adds the @defvar and also an
introductory paragraph in that node.

gdb/doc/ChangeLog
2019-03-20  Tom Tromey  <tromey@adacore.com>

	* python.texi (Selecting Pretty-Printers): Use @defvar for
	gdb.pretty_printers.