Manual review: Chacha20

Manual review: Chacha20

I intended this to be a fairly light review, but this ended up being a
rather comprehensive rewrite...

I tried to follow the advice of mdoc(7) as much as possible.  CuleX
did a remarkable job adapting the old manual to man pages without
butchering the original text; but I now think it has to be butchered
eventually.

Expanded the EXAMPLES section.  I think it gives a clearer view of all
possible use cases that way.

Replaced STANDARDS by IMPLEMENTATION DETAILS.  The choice of primitive
has implications for the end users, and thus isn't a mere
implementation detail.  Also serves to emphasise that Monocypher does
implement widely reviewed standards, as opposed to home-invented
crypto.

Replaced CAVEATS by SECURITY CONSIDERATIONS.  Arguably, either section
would do.  I changed it because every consideration listed there would
trigger a vulnerability if not observed.