text-input: Rename text-input to text_input

Interface names are lower_snake_case, and corresponding descriptions
should match, for accuracy and clarity. This renaming only affects
description text, to follow the convention that exists elswhere in
this protocol document and in other protocol docs, when referring to
interface names.

Signed-off-by: Yong Bakos <ybakos@humanoriented.com>
Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Jonas Ådahl <jadahl@gmail.com>