ARM: 7816/1: CONFIG_KUSER_HELPERS: fix help text

Commit f6f91b0d9fd9 ("ARM: allow kuser helpers to be removed from the
vector page") introduced some help text for the CONFIG_KUSER_HELPERS
option which is rather contradictory.

Let's fix that, and improve it a little.

Change-Id: I9059789c8404556937c5f5d0fa15619f0ead6460
Cc: <stable@vger.kernel.org>
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Git-commit: ac124504ecf6b20a2457d873d0728a8b991a5b0c
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git
CRs-fixed: 561044
Signed-off-by: Joonwoo Park <joonwoop@codeaurora.org>