Document configure only should be called once

[slominskir]

Fixes https://github.com/confluentinc/schema-registry/issues/1903

[OneCricketeer]

I don't think a Javadoc comment will save anyone from accidentally calling the configure method more than once.

If the purpose is to prevent that from happening, why not throw IllegalStateException, for example?

[slominskir]

Documenting the expected behavior is better than nothing. An Exception is more direct, but also more invasive and risky to implement - that configure method is inherited and overridden all the way up to AbstractKafkaAvroDeserializer and AbstractKafkaAvroSerializer. Where would this Exception be thrown from and how is the called state maintained? I figured just documenting the current behavior was good enough, but throwing an Exception sounds good if someone else wants to spend the time to write it and test it.

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