RAIDZ.rst: add note about usable space (openzfs#477)

Author: jameskimmel

docs/Basic Concepts/RAIDZ.rst

@@ -51,7 +51,7 @@ we will allocate on disk:
 
 - one 4K block of data
 
-- one 4K padding block
+- one 4K parity block
 
 and usable space ratio will be 50%, same as with double mirror.
 
@@ -62,7 +62,9 @@ Another example for ``ashift=12`` and ``recordsize=128K`` for raidz1 of 3 disks:
 
 - one stripe can have up to 2 data parts of 4K size because of 1 parity blocks
 
-- we will have 128K/2 = 64 stripes with 8K of data and 4K of parity each
+- we will have 128K/8k = 16 stripes with 8K of data and 4K of parity each
+
+- 16 stripes each with 12k, means we write 192k to store 128k
 
 so usable space ratio in this case will be 66%.