mismatched replication level: both 3-way and 2-way mirror vdevs are present

System information

Type	Version/Name
Distribution Name	Centos
Distribution Version	7
Kernel Version	5.5
Architecture	x86_64
OpenZFS Version	2.1.99-1

Describe the problem you're observing

Adding mirrored special devices results in an error when redundancy level does not match the target pool. Either the manpage / documentation is incorrect or the implementation is.

Describe how to reproduce the problem

#
# zpool create tank mirror $p/tank_DISK00 $p/tank_DISK01 $p/tank_DISK02 \
   special mirror $p/tank_SPECIAL00 $p/tank_SPECIAL01 
invalid vdev specification
use '-f' to override the following errors:
mismatched replication level: both 3-way and 2-way mirror vdevs are present
#
# zpool create tank mirror $p/tank_DISK00 $p/tank_DISK01 $p/tank_DISK02 \
   special mirror $p/tank_SPECIAL00 $p/tank_SPECIAL01 $p/tank_SPECIAL02
# 
 

Manpage states:

 special
         A device dedicated solely for allocating various kinds of internal metadata,
         and optionally small file blocks.  The redundancy of this device **should** match
         the redundancy of the other normal devices in the pool. If more than one spe‐
         cial device is specified, then allocations are load-balanced between those
         devices.

Which would imply it is optional.