Update comments to target_can_use_hardware_watchpoint

Update comments to target_can_use_hardware_watchpoint

The return value of target_can_use_hardware_watchpoint isn't well
documented, so this patch is to update the comments to reflect the
fact.  This patch also removes a trailing ";" which is picked up
from Pedro's patch https://sourceware.org/ml/gdb-patches/2015-04/msg00527.html

gdb:

2015-04-17  Yao Qi  <yao.qi@linaro.org>
	    Pedro Alves  <palves@redhat.com>

	* target.h (target_can_use_hardware_watchpoint): Update comments.
	Remove trailing ";".