Update and clarify comments.  Remove the useless counter for impossible, but
seen in wild situation (on buggy hypervisors).

In collaboration with:	bde
MFC after:	1 week


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271924 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f