pause.3: miscellaneous rewrites, cleanup

pause.3: miscellaneous rewrites, cleanup

Eliminate some redundant or extraneous pieces from the pause.3 page.
Say the "thread" "blocks", don't say the "process" "pauses".  No need
to enumerate the ways a signal can be delivered.  Add a few relevant
cross-references.

With input from millert@ and schwarze@.

Link: https://marc.info/?l=openbsd-tech&m=166801212316670&w=2
ok millert@ schwarze@