growfs(7): clarify assumptions and limitations

Document that the growfs(7) script works only if the root file system
is in the last partition and free space immediately follows it.
Don't imply that /usr can be a separate partition, as that would
likely mean that root is not last.

Reported by: marklmi at yahoo dot com

(cherry picked from commit 8664e266d6a4573d1875ee8f4ce0cdb091171780)