check-qjson: Document we expect invalid UTF-8 to be rejected

The JSON parser rejects some invalid sequences, but accepts others
without correcting the problem.

We should either reject all invalid sequences, or minimize overlong
sequences and replace all other invalid sequences by a suitable
replacement character.  A common choice for replacement is U+FFFD.

I'm going to implement the former.  Update the comments in
utf8_string() to expect this.

Signed-off-by: Markus Armbruster <[email protected]>
Reviewed-by: Eric Blake <[email protected]>
Message-Id: <[email protected]>

Author: Markus Armbruster