smr: Remove the return value from smr_wait()

This is supposed to be a blocking version of smr_poll(), so there's no
need for a return value. No functional change intended.

MFC after: 1 week

man9: Add an smr(9) manual page

Also update the UMA manual page to mention its SMR-enabled
functionality, and update locking.9 to mention both epoch and SMR.
Details of its usage are provided in the SMR manual page.

Reviewed by: Olivier Certner, mhorne, kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D38108