docs(appconfig): better docstring for app config content type

[jeqcho]

Issue # (if applicable)

Closes #35093.

Reason for this change

Improve doc string to clarity what inputs are accepted for contentType.

Description of changes

Simple documentation additions.

Describe any new or updated permissions being added

None

Description of how you validated changes

None, only documentation changes.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license

[aws-cdk-automation]

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[aws-cdk-automation]

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[aws-cdk-automation]

Your pull request must be based off of a branch in a personal account (not an organization owned account, and not the main branch). You must also have the setting enabled that allows the CDK team to push changes to your branch (this setting is enabled by default for personal accounts, and cannot be enabled for organization owned accounts). The reason for this is that our automation needs to synchronize your branch with our main after it has been approved, and we cannot do that if we cannot push to your branch.