newfs_msdos.8: example for specific cluster size

The usual use case in 2024 for newfs_msdosfs is creating filesystems on SD cards
for older hardware. In most tutorials, they call the cluster size "allocation
size". Therefore, add a small note next to cluster size that it is also called
allocation size, and add an example for how to do this.

Reviewed by: jhb
Pull Request: https://github.com/freebsd/freebsd-src/pull/1045