manual: make setrlimit() description less ambiguous

The existing description for setrlimit() has some ambiguity. It could be
understood to have the semantics of getrlimit(), i.e., the limits from the
process are stored in the provided rlp pointer.

Make the description more explicit that rlp are the input values, and that
the limits of the process is changed with this function.

Reviewed-by: Florian Weimer <fweimer@redhat.com>
(cherry picked from commit aedbf088)