Clarify avro support in docs

[rmoff]

On https://github.com/confluentinc/ksql/blob/4.1.x/docs/faq.rst#can-i-use-ksql-with-my-favorite-data-format-e-g-json-avro

1. so we should be removing “Support for Apache Avro is expected soon.“
2. add a clear note that support for Avro is for message values only, and that Avro keys are not supported (c.f. Add support for generic message keys (e.g. message keys in Avro or JSON format, STRUCT) #824 )

[miguno]

1. so we should be removing “Support for Apache Avro is expected soon.“

This has already been fixed since.

2. add a clear note that support for Avro is for message values only, and that Avro keys are not supported (c.f. Add support for generic message keys (e.g. message keys in Avro or JSON format, STRUCT) #824 )

@joel-hamill : Could you take a look at that? This should be mentioned not just in the FAQ but also in the CREATE STREAM/TABLE related docs, and in the specific Avro/Schema Registry section. Also, we can add the same note about JSON in the CREATE STREAM/TABLE related docs.

[miguno]

Btw, KSQL does have some (limited) support for keys in JSON format, see @rodesai's answer at https://stackoverflow.com/questions/49844380/ksql-kafka-formatting-of-keys.

[joel-hamill]

@rmoff @miguno see #1152