No multiple distributions in Distribution, update footnote

No multiple distributions in Distribution, update footnote

Document that the Debian archive software does not allow listing
multiple distributions in the *.changes file.  Update the footnote
listing Distribution field values to list the ones currently used,
remove frozen, remove the discussion of testing, and add a mention
of stable-security and testing-security.

Based on wording proposed by Colin Watson and Adam D. Barratt.