Docs: Increase detail about necessary options for VPC instance creation

VPC instances will need --server-connect-attribute, especially when the
VPC instance is created with --associate-eip on the command line.
Without it, the SSH process in the knife ec2 server create will insist
on connecting to the original public IP address, rather than the new
EIP
Merge pull request #203 from opscode/sh/create-server-sca

Docs: Increase detail about necessary options for VPC instance creation