zed: reap child after killing on time-out

When a child process is killed waitpid() must be called on the
pid the reap the zombie process.

Update BUGS section to reflect reality by replacing "zedlets
aren't time limited with "zedlets can be interrupted".

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Closes #11769
Closes #11798