tty/vt: Corrected doc of vc_sanitize_unicode(), vc_translate_unicode()

tty/vt: Corrected doc of vc_sanitize_unicode(), vc_translate_unicode()

Corrected the doc of vc_sanitize_unicode() and vc_translate_unicode(),
tightly coupled functions which parse UTF-8 byte sequences.
1. Desc. of @rescan corresponded to the meaning of the return value -1.
   Corrected + added "Return:" section.
2. Replaced the ambiguous "character" with "code point" or "byte".

Signed-off-by: Roman Žilka <roman.zilka@gmail.com>
Link: https://lore.kernel.org/r/bee9faa8-0ea7-4411-bf77-3cb2e06385c7@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>