Jumbo update of installation notes for alpha:

Jumbo update of installation notes for alpha:
- mention more models supported by each floppy
- mention more supported and unsupported models
- sync cdrom boot instructions with reality
- update ftp urls, ftp.digital.com no longer exist (but I resisted whining
  at HP which does not provide all the contents formerly available from
  ftp.digital.com and which you have to pick from educational sites which
  have mirrored them)
- various little rewording

with help from sobrado@