(aws-ssm): Clarification on SSM Parameter Overwriting Needed

There's a lot of opacity around how, when, and by what mechanism `StringParameter` constructs overwrite previous, existing, or modified parameter values. Through our own experiments, we know the following conditions exist:

- A parameter value is written when the param is first created
- If the value for a previously created param is manually updated (via aws console or aws cli), cdk does not update the value on subsequent deploys
- If the value for a previously created param is manually updated (via aws console or aws cli), but the value is changed in the cdk code for the param, cdk _does_ update the value on subsequent deploys, overwriting it with the value in the cdk code.

I'm unable to find any documentation or explanation as to the mechanisms that determine when CDK updates a parameter value, and under what conditions. Source diving the ssm package didn't reveal any clues either. This is currently "black box" or "magic" and it goes without saying that's not great.

I'd love to see some information around this and the mechanisms involved (including if metadata is involved, and where that metadata lives), even if only as a reply to this issue. Official documentation would of course assist the entire userbase.

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This is a 📕 documentation issue

Start by reading the SSM package and comparing its behavior with the three deployment experiments described in the issue. Document when StringParameter values are written or overwritten, including whether metadata is involved and where it lives; done means the behavior and mechanisms are explained clearly in official documentation.

Written by the indexing model from the issue text.