make a sentence better

add CONTEXT

these can all be called during autoconf, in process context, or from
interrupts.

better history.

atomic.9 doesnt exist to Xr to now.

fix Dt lines so they match the page name.

reminded by jmc@

rename the atomic_foo manpages to atomic_foo_int or _uint cos we name these
after functions. for reasons.

reminded by jmc@