py-linetable.c: Fix doc of LineTable.source_lines' return type

py-linetable.c: Fix doc of LineTable.source_lines' return type

The ltpy_get_all_source_lines function, use to implement
the gdb.LineTable.source_lines method, returns a list:

    source_list = PyDict_Keys (source_dict);
    return source_list;

This patch fixes the function's documentation as well as its docstring
to say that it returns a list rather than a FrozenSet.

gdb/ChangeLog:

        * py-linetable.c (ltpy_get_all_source_lines): Adjust function
        documentation to say that it returns a list rather than
        a FrozenSet.
        (linetable_object_methods): Update the docstring of the
        "source_line" entry.

Tested on x86_64-linux.