aws-sdk-php, IamClient - How to update policy json, the 'PolicyDocument'?

https://docs.aws.amazon.com/aws-sdk-php/v3/api/class-Aws.Iam.IamClient.html

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The "role" section makes sense to me.

There is createRole, getRole, deleteRole, and updateRole. Also, the getRole function returns the 'AssumeRolePolicyDocument', so I could compare it to what I want it to be and update it if necessary with the updateRole function.

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The "policy" section seems to be missing functionality

There is createPolicy, getPolicy, and deletePolicy - but there is no updatePolicy?

When you run 'createPolicy', you pass in the 'PolicyDocument' as json. But the 'getPolicy' function does not return the 'PolicyDocument' so I can confirm it's correct. I don't see how to query this information. There also isn't a getPolicyDocument function.

And if I was able to get the PolicyDocument json, how would I update the Policy? Is there a function that I'm overlooking?

Or am I logically approaching this problem incorrectly?