Fix gdb.Field attributes documentation for enum types.

Fix gdb.Field attributes documentation for enum types.

The following patch ...

    | commit 14e75d8ea4fe9ed4dbf292ae4a9745e33e2ff353
    | Date:   Wed Apr 18 06:46:47 2012 +0000
    |
    |     gdb/
    |         PR symtab/7259:
    | [...]

... discussed under ...

    [PATCH] Allow 64-bit enum values
    http://www.sourceware.org/ml/gdb-patches/2012-03/msg00772.html

... introduced a change in the gdb.Fields API without documenting it:

    | I took a separate approach from the one I took in:
    |
    | http://sourceware.org/ml/gdb-patches/2012-02/msg00403.html
    |
    | and removed the overloaded meaning of the bitpos location variable to
    | fix PR symtab/7259. In the following patch, I introduce a separate
    | field_location union member 'enumval' which can accept LONGEST and
    | hence expand enum values to 64-bit signed values. With this change,
    | bitpos now only is used for (non-negative) offsets into structures,
    | since the other overload of bitpos (range bounds) were already
    | separated into struct range_bound.

This patch updates the documentation to reflect that change.

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.