where.adoc (Get FreeBSD, Download FreeBSD): corrections and improvements

Supplemental to D36520 https://reviews.freebsd.org/D36520:

• a few corrections (for example, the general advice about mirror sites was misplaced under FreeBSD 12.3-STABLE)
• narrower column headings should improve tables on smaller displays
• a few other improvements
• generally less verbose.

Reviewed by: carlavilla (mentor)
Approved by: carlavilla
Pull request: https://github.com/grahamperrin/freebsd-doc/pull/4
Differential revision: https://reviews.freebsd.org/D36523