Documentation/kbuild: add info that 'choice' can have a symbol name

Changeset 5a1aa8a1

 added the possibility to name a choice,
and to have the same choice be defined multiple times.
But the documentation was forgotten, so this updates it accordingly.

Thanks to Arnaud Lacombe for pointing it to me in the first place:
  http://www.spinics.net/lists/linux-kbuild/msg03940.html

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Cc: Roman Zippel <zippel@linux-m68k.org>
Cc: Arnaud Lacombe <lacombar@gmail.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>