MFC r286701:

MFC r286701:

  If a specific timecounter has been chosen via sysctl, and a new
  timecounter with higher quality registers (presumably in a module that has
  just been loaded), do not undo the user's choice by switching to the new
  timecounter.

  Document that behavior, and also the fact that there is no way to
  unregister a timecounter (and thus no way to unload a module containing
  one).

git-svn-id: svn://svn.freebsd.org/base/stable/10@287042 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f