vfs: vntblinit(): Rework comments on the computation of 'kern.maxvnodes'

Remove the parts that describe what is already in the code formula. Add
hints about which of 'physvnodes'/'virtvnodes' takes precedence, and
when 'desiredvnodes' becomes smaller than 'maxfiles'. These were both
computed analytically and verified experimentally.

Reviewed by: kib (older version)
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D50310

(cherry picked from commit bf8890c84bacd8b1acebb2cb9a0944b1bb102b68)