Clean up confusing comment.  Move it to the place of code which is
talked about.  Explain where the mentioned trampoline located
(usermode), and the fact that attempt to exit last thread is denied in
kernel (by delegating the work to usermode).

Sponsored by:	The FreeBSD Foundation
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@274038 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f