mke2fs: document bigalloc and cluster-size

Bigalloc feature has been used for a long time, but the documentation
in mke2fs is still missing.  So add it.

Addresses-Debian-Bug: #669730

Signed-off-by: Zheng Liu <wenqing.lz@taobao.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
1 file changed