Add zfs_blake3_impl to zfs.4

The zfs module parameter zfs_blake3_impl got no manual page entry while
adding BLAKE3 to OpenZFS. This commit adds the required notes about the
parameter into zfs.4

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Co-authored-by: Ryan Moeller <ryan@freqlabs.com>
Signed-off-by: Tino Reichardt <milky-zfs@mcmilk.de>
Closes openzfs#13725

man/man4/zfs.4

@@ -1168,6 +1168,20 @@ Selecting any option other than
 results in vector instructions
 from the respective CPU instruction set being used.
 .
+.It Sy zfs_blake3_impl Ns = Ns Sy fastest Pq string
+Select a BLAKE3 implementation.
+.Pp
+Supported selectors are:
+.Sy cycle , fastest , generic , sse2 , sse41 , avx2 , avx512 .
+All except
+.Sy cycle , fastest No and Sy generic
+require instruction set extensions to be available,
+and will only appear if ZFS detects that they are present at runtime.
+If multiple implementations of BLAKE3 are available, the
+.Sy fastest will be chosen using a micro benchmark. You can see the
+benchmark results by reading this kstat file:
+.Pa /proc/spl/kstat/zfs/chksum_bench .
+.
 .It Sy zfs_free_bpobj_enabled Ns = Ns Sy 1 Ns | Ns 0 Pq int
 Enable/disable the processing of the free_bpobj object.
 .