A first go round at comprehensive docs. 

Although tags can be applied to many resource types, there are a number of types which cannot be tagged (e.g. `AWS::EC2::EIP, AWS::EC2::EIPAssociation`, `AWS::EC2::SecurityGroupEgress`, `AWS::EC2::SecurityGroupIngress`, `AWS::EC2::SubnetNetworkAclAssociation`, `AWS::EC2::SubnetRouteTableAssociation`). 

I think we should modify this text to explain that while many resources support tags, users should reference the documentation to be sure.