procfs: Document as deprecated

Update the procfs(5) man page to clarify that it's deprecated.

Reviewed By: bcr, 0mp (earlier version)
Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D22275

(cherry picked from commit 70774c637b87eac1d9a94bb960e6f5242dad9e43)