sim: Fix API comments for optParamIn.

sim: Fix API comments for optParamIn.

The top level comment was correct, but the parameter comments talked
about writing parameters instead of reading them. Also simplified the
wording of the return value comment.

Change-Id: I156aba5b69c281ee2f34297bf3f75fd0acfb2b6e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36278
Maintainer: Gabe Black <gabe.black@gmail.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>