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

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

(cherry picked from commit 4e240e55d8186b44fe1bcda09b90ca7ffe65a58b)