Consider Making update-domain-association Less Destructive

**Is your feature request related to a problem? Please describe.**

It's hard to tell by the naming of this command that you can't just update one entry of the domain associations rules. If you only try to update the one association you want, you end up wiping out the other associations that exist for that project. This seems too aggressive for an update. And is not well documented enough to prevent unsuspecting users from potentially bringing down their prod site because they would wipe out that association with this command.

**Describe the solution you'd like**

Update `update-domain-association` to only update the `--sub-domain-settings` that are passed and preserve the existing associations not passed to the command.

At the very least, just update the documentation so that users know what kind of outcomes to expect from this command.

**Additional context**

https://github.com/aws-amplify/amplify-console/issues/196#issuecomment-548053431
https://docs.aws.amazon.com/cli/latest/reference/amplify/update-domain-association.html

Start with the `update-domain-association` command, its `--sub-domain-settings` behavior, and the AWS CLI reference linked in the issue; review the related issue comment for existing context. Done means either preserving associations not passed to the command or clearly documenting the current destructive outcome and its risks.

Written by the indexing model from the issue text.