gdb/doc: fix the example for get_set_string in Python API docs

The small example for gdb.Parameter.get_set_string does not return a
string.  The documentation is very clear that this method must return
a string, and indeed, inspecting the code in gdb/python/py-param.c
shows that a string return value is required (if an exception is not
thrown).

While inspecting the code in gdb/python/py-param.c I noticed that the
comment for the C++ code that invokes the Python get_set_string method
is wrong, so I updated that too.

gdb/ChangeLog:

	* python/py-param.c (get_set_value): Update header comment.

gdb/doc/ChangeLog:

	* python.texinfo (Parameters In Python): Return empty string in
	small example code.