No documentation of `stsClient` method for StsAssumeRoleCredentialsProvider.Builder

### Describe the issue

It appears that in order to use the `StsAssumeRoleCredehntialsProvider.Builder` you need to call an `stsClient` method to set the client that it uses. Failing to do so results in a `NullPointerException`.

However, that method is not mentioned at all in the documentation for the Builder class or the credentials provider class. I was only able to discover the need for calling this method, due to a [stack overflow answer](https://stackoverflow.com/questions/62921548/aws-sdk-2-assume-role) and the source code for [`StsCredentialsProvider`](https://github.com/aws/aws-sdk-java-v2/blob/master/services/sts/src/main/java/software/amazon/awssdk/services/sts/auth/StsCredentialsProvider.java).

### Links

https://sdk.amazonaws.com/java/api/latest/software/amazon/awssdk/services/sts/auth/StsAssumeRoleCredentialsProvider.Builder.html