Update "minimum requirements" FAQ to reflect current reality regarding
memory usage, including the fact that installing FreeBSD needs more
memory than actually running it. Rearrange the text a little bit to
make some more sense.

This FAQ is very i386-centric; there is no mention of what the minimum
requirements are for !i386 platforms.

The need for this change was...

Pointed out by: simon
Inspired by: PR docs/77304