Update documentation of `DefaultAWSCredentialsProviderChain`.

[teo-tsirpanis]

Issue #, if available:

Description of changes:

This PR updates the documentation of DefaultAWSCredentialsProviderChain to match the credential sources it uses. https://github.com/aws/aws-sdk-cpp/blob/a95671dc38b2c61a18f5398d1e549278e1fbe440/src/aws-cpp-sdk-core/source/auth/AWSCredentialsProviderChain.cpp#L45-L49

Check all that applies:

• [X] Did a review by yourself.
• [ ] Added proper tests to cover this PR. (If tests are not applicable, explain.)
• [ ] Checked if this PR is a breaking (APIs have been changed) change.
• [ ] Checked if this PR will not introduce cross-platform inconsistent behavior.
• [ ] Checked if this PR would require a ReadMe/Wiki update.

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• [ ] Linux
• [ ] Windows
• [ ] Android
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• [ ] IOS
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