[RFA] Update manual for Rust change

[Eli Zaretskii]

> From: Tom Tromey <tom@tromey.com>
> Cc: Tom Tromey <tom@tromey.com>
> Date: Fri,  1 Dec 2017 14:17:37 -0700
> 
> I realized today that a recent change to the Rust support required an
> update to the manual; and so I updated NEWS as well.
> 
> 2017-12-01  Tom Tromey  <tom@tromey.com>
> 
> 	* NEWS: Mention Rust trait object inspection.
> 
> 2017-12-01  Tom Tromey  <tom@tromey.com>
> 
> 	* gdb.texinfo (Rust): Update trait object status

OK.