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.

gdb/python/py-param.c

@@ -368,11 +368,10 @@ call_doc_function (PyObject *obj, PyObject *method, PyObject *arg)
 }
 
 /* A callback function that is registered against the respective
-   add_setshow_* set_doc prototype.  This function will either call
-   the Python function "get_set_string" or extract the Python
-   attribute "set_doc" and return the contents as a string.  If
-   neither exist, insert a string indicating the Parameter is not
-   documented.  */
+   add_setshow_* set_doc prototype.  This function calls the Python function
+   "get_set_string" if it exists, which will return a string.  That string
+   is then printed.  If "get_set_string" does not exist, or returns an
+   empty string, then nothing is printed.  */
 static void
 get_set_value (const char *args, int from_tty,
 	       struct cmd_list_element *c)