docs(cognito_identity_credentials): Explain limitation of CognitoIden…

[RanVaknin]

This method uses the simplified authflow, which means users cannot explicitly set a session policy. This results in STS appending the Default Session Policy to the request, which can lead to limiting permissions even if they are explicitly attached to a role.

The doc change highlights the limitation of the connivence method since it uses that Enhanced (simplified) flow "under the hood" which limits permissions on the assumed role because of the intersection of multiple policies resulting in the least permissive one being applied to the credentials returned from STS.

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