Amend docs with `immutable` property attribute.

Include warning about updates not being supported for a propety
based on its `immutable` value being True.

Change-Id: I10fe495a2a35429b3d6e254c23ae91732e2e4dd8
Implements: blueprint implement-aws-updates-not-supported
Deny updates to AWS::EC2::Volume resource

According to AWS docs [1] Volume resource does not support updates
(including resource replacement) of any of its properties.
To align our AWS-compatible resource with such behavior, all properties
of AWS::EC2::Volume resource are marked as immutable.

[1] http://docs.aws.amazon.com/AWSCloudFormation/latest/UserGuide/aws-properties-ec2-ebs-volume.html

Change-Id: I7fea68f11c58d395c1eb31fae834afcab5dc4d1e
Closes-Bug: #1340093
Merge "Amend docs with `immutable` property attribute."
Merge "Deny updates to AWS::EC2::Volume resource"