Document additional -c caveat

One might expect "send data as it is on disk, and cannot trigger
compression changes" to imply "does not attempt to compress data
that was not compressed on the sender."

One would be mistaken.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12570