[Task]: Document Methods in the HealthcareApiClient

[svetakvsundhar]

What needs to happen?

Several methods are not documented in the Healthcare API Client (https://github.com/apache/beam/blob/master/sdks/java/io/google-cloud-platform/src/main/java/org/apache/beam/sdk/io/gcp/healthcare/HealthcareApiClient.java#L238).

We should add documentation for these methods (regarding DICOM IO).

Issue Priority

Priority: 3 (nice-to-have improvement)

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[svetakvsundhar]

.take-issue