Rewrite the description of the .Os macro to match reality.

Rewrite the description of the .Os macro to match reality.

While BSD releases from the CSRG did indeed put "BSD" and an operating
system version number on .Os macro lines, that practice was discontinued
in OpenBSD 25 years ago because it causes maintenance effort without
providing any benefit.
Other operating systems tend to agree that .Os is usually best left blank.
For example, FreeBSD weeded out .Os arguments about two years after OpenBSD.