kqueue.2: Editorial pass

• Use consistent language to describe user values unchanged by the kernel.

• Replace passive language with active in a few places.

• Add a history note for kqueuex() and kqueue1().

• Add an MLINK and synopsis for kqueue1().

• Various wording and markup tweaks.

Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D48203