[RFA/doco] Fix gdb.Field attributes documentation for enum types.

[Eli Zaretskii]

> From: Joel Brobecker <brobecker@adacore.com>
> Date: Mon, 23 Dec 2013 11:23:06 +0400
> 
> gdb/doc/ChangeLog:
> 
>         * gdb.texinfo (Types In Python): Fix the documentation of
>         attribute "bitpos" in class gdb.Field for enum types.  Add
>         documentation for attribute "enumval" in that same class.
> 
> Tested on x86_linux. OK to check in?

I guess so, although I'm not sure the reader will always understand
the exact meaning of "bit position of the field".  Maybe we should
explain some more?

Thanks.