Adding paragraph on updates to security instances section

Added paragraph noting that instances will need to be updated separately
from hypervisors, and cleaning up grammar/voice. Also adding additional
paragraph to clarify difference between hypervisors and guest vms.
Fixed my typos.

Change-Id: If3f5cc82e45f312187c6c5a6d277f1be508348e3
Partial-Bug: #1344342
Merge "Adding paragraph on updates to security instances section"