tc_init.9: miscellaneous cleanup and rewrites

tc_init.9: miscellaneous cleanup and rewrites

- In DESCRIPTION, try to more fully describe what kern_tc.c does.
  Clean up the wording.

- Mention *all* the requirements for timekeeping hardware.  Describe
  the rollover margin in plainer language.

- Revise field descriptions for struct timecounter.  Don't mention
  fields the driver doesn't need to initialize.  Document the tc_user
  field.

- Add a CONTEXT section.

- In SEE ALSO, switch to an https URI on the main freebsd.org website.

- In HISTORY, note that the timecounting code first reached end users
  in FreeBSD 3.0.  This commit is probably the first one:

"Replace TOD clock code with more systematic approach."
https://cgit.freebsd.org/src/commit/sys/sys/timetc.h?id=7ec73f64179417aeda085c1c338385559fb49c23

- Add an AUTHORS section.

With input from Poul-Henning Kamp.

Link: https://marc.info/?l=openbsd-tech&m=168004968214914&w=2
ok jmc@