xfsprogs: mkfs manpage fix for -nsize/log

xfsprogs: mkfs manpage fix for -nsize/log

There are two limitations for the mkfs.xfs -nsize/log option:
1) directory block size must be a power of 2.
2) it can't be less than a file system block size.

Current man page don't include the above information. User could
be confused with errors, say "Illegal value xxx for -n size option", but
they can't find out the cause by checking the man page.

The patch adds the two limitations to the manpage.

Signed-off-by: Wengang Wang <wen.gang.wang@oracle.com>
Reviewed-by: Eric Sandeen <sandeen@sandeen.net>