vt: add comments for KDMKTONE ioctl implementation

The KDMKTONE ioctl, introduced in commit 916347f77e6c, is used to beep
the PC speaker. For historical reasons the frequency is specified as an
8254 PIT divisor for a 1.19MHz clock. Linux provides this same ioctl.
Add a comment to vtterm_beep to avoid someone wanting to "fix" this in
the future.

Also add an XXX comment that the period unit is supposed to be "timer
ticks." Note that nothing in the base system uses this ioctl.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D47395