Small fixes to the Python API doc

Small fixes to the Python API doc

First:
"Breakpoint.delete" is missing parenthesis.

Second:
Someone on IRC asked, how come there is no disable() method in the
Breakpoint object.  It turns out you have to do "bp.enabled = False".
Since every normal person would probably search for "disable" in that page
if their intent is to disable a python breakpoint, I thought it would be
useful if the description contained "disable" so it would be easy to find.
The result might seem a bit silly and redundant, so I am open to
suggestions.

gdb/doc/ChangeLog:

	* python.texi (Breakpoints In Python): Add parenthesis after
	Breakpoint.delete.  Clarify Breakpoint.enabled description so
	that it	contains "disable".