pmc_allocate(3): document the count argument

This was added in b2ca2e50b9aa, and serves to provide an initial value
to the PMC, eliminating the need for a second syscall via pmc_set(3).

Reviewed by: gnn, 0mp (manpages)
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29885