Improve doc on how to execute a Python script from GDB.

Improve doc on how to execute a Python script from GDB.

        * gdb.texinfo (File Options): Adjust the documentation of this
        switch to refer to the "source" command rather than partially
        duplicating some of the relevant information.
        (Extending GDB): Introduce and document the set/show script-extension
        setting.
        (Command Files): Add note explaining that the "source" command
        is also used to evalute scripts written in other languages.
        Remove the short slightly incorrect reference to sourcing Python
        scripts.
        (Python Commands): Document how to execute a Python script from GDB.