Cleanup special/dedup language

Cleanup special/dedup language

This standardizes the language on "deduplication tables" rather than
"dedup data" (which might be read as the data blocks rather than the
DDT).  Likewise, it standardizes on "small file blocks".  It also
standardizes on "normal" rather than using both "normal" and "general"
in the same paragraph. I also replaced "non-specified" with the more
explicit "non-dedup/special".

Reviewed-by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Richard Laager <rlaager@wiktel.com>
Closes #8713