header_rewrite: set-redirect documentation corrections

set-redirect requires two params status-code and destination. Also
QSA appears to be a supported operator flag. This closes #98
header_rewrite: remove docs references to PARAMS and PORT conditions

PARAMS doesn't appear to be implemented... removing it from the
docs. Was it replaced by QUERY? PORT doesn't appear to be supported
in ATS 5 either.

This closes #102
docs: Minor edits

This closes #89