This improves how Parameter set/show documentation is processed
and allows for more dynamic content.
+ ** Symbols, Symbol Table, Symbol Table and Line, Object Files,
+ Inferior, Inferior Thread, Blocks, and Block Iterator APIs now
+ have an is_valid method.
+
** Breakpoints can now be sub-classed in Python, and in particular
you may implement a 'stop' function that is executed each time
the inferior reaches that breakpoint.