Clarify description of zfs_special_class_metadata_reserve_pct (#490)

Based on spa_misc.c L1974 the codepath treats DDT objects the same
as regular metadata, so the language in the docs regarding "prevents
small blocks or dedup table from completely consuming" was changed
to clarify this

docs/Performance and Tuning/Module Parameters.rst

@@ -8249,7 +8249,7 @@ zfs_special_class_metadata_reserve_pct
 
 ``zfs_special_class_metadata_reserve_pct`` sets a threshold for space in
 special vdevs to be reserved exclusively for metadata. This prevents
-small blocks or dedup table from completely consuming a special vdev.
+small data blocks from completely consuming a special vdev.
 
 ====================================== ================================
 zfs_special_class_metadata_reserve_pct Notes