growfs(7): document addition of swap partition and growfs_fstab script

Add documentation of the growfs script's new ability to add a swap
partition, expanding on the previous functionality as well. Add the
growfs_fstab helper script, which runs separately. Add a description
of how to expand a file system a second time if swap had been added.
While here, fix a typo.

Reviewed by: pauamma
Differential Revision: https://reviews.freebsd.org/D37465