Update aws-properties-codedeploy-deploymentgroup-deployment-revision.md

[ZainubAmod]

RevisionType supporting "AppSpecContent" and "String" values has created customer confusion, frustration and has lead to the creation of support cases since revision type accept these values but there is no way on how to add these values. A note specifying that this is not supported (or the removal of the values entirely) will aid in removing customer confusion

Issue #, if available:

Description of changes:

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