Document SSM agent limitations

[stmcginnis]

What I'd like:

The AWS variants include the SSM agent for accessing the console interface and retrieving node information. There are additional commands users can run via the aws ssm command line. Some of these commands are relevant for Bottlerocket and some are not.

AWS-UpdateSSMAgent is one that doesn't work since Bottlerocket does not allow packages to be updated for a running instance. There are likely more. AWS-RunPatchBaseline appears to be another one.

Rather than leaving it up to our users to figure out what works and what doesn't, we should provide some documentation that lets users know what to expect. This doesn't need to be an exhaustive list of SSM agent commands. That is better documented elsewhere. But we should at least point out some of the main commands or type of commands that users should be aware are not relevant when working with Bottlerocket.