add what a jobspec of just plain "%" means to the manual

[Dan Jacobson]

The manual doesn't seem to say what a jobspec of just "%" means.  It
only mentions % with some other chars attached to its back.  So, I'd
explain what jobspec of plain "%" refers to, lest the user think it
will always refer to the latest background job or something.

$ sh -c 'trap echo\ hi 0;sleep 44'&
[3] 3923
$ kill %
[2]+  Terminated              less tmp/bash-man
$ jobs
[1]+  Stopped                 TERM=vt100 lynx
[3]-  Running                 sh -c 'trap echo\ hi 0;sleep 44' &

"kill %", unexpectedly for me at least, killed job 2 here.
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