qapi: Refill doc comments to conform to current conventions

qapi: Refill doc comments to conform to current conventions

For legibility, wrap text paragraphs so every line is at most 70
characters long.

To check the generated documentation does not change, I compared the
generated HTML before and after this commit with "wdiff -3".  Finds no
differences.  Comparing with diff is not useful, as the refilled
paragraphs are visible there.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-ID: <20240322140910.328840-11-armbru@redhat.com>