mkfs.xfs man page for -nsize

[Wengang Wang]

Hi experts,

When trying -nsize=128 and -n6144 option to mkfs.xfs respectively, I got
error message:
"Illegal value xxx for -n size option"

But checking the man page,

----------------
-n naming_options
	These  options  specify  the  version  and  size parameters  for the
naming (directory) area of the filesystem. The valid nam-ing_options are:

	        size=value | log=value
			The block size is specified either as a value in bytes
with size=, or as a base two logarithm value with log=.  The default size value
for version 2 directories is 4096 bytes (4 KiB), unless the filesystem block
size is larger than 4096, in which case the default value is the filesystem
block size.  For version 1 directories the  block  size  is the same as the
filesystem block size.
----------------

I got no idea why the value is illegal.

Checking the codes, I found it also must be larger or equal to block size and it
must be in powers of 2, 2^N. Should we add these two limitation to man page?

regards,
wengang.