[hotfix][docs] mention kryo in serialization and link to page

[robstoll]

What is the purpose of the change

improve documentation

Brief change log

I am new to flink so maybe I misunderstood something. As far as I am get Avro is only used if the POJO comes from a source which was deserialised with Avro. Otherwise Kryo is used. Thus I changed the text, mentioning Kryo as well and added a link to the Serialization of POJO types section (I guess the link to the other page makes sense in all cases).

Verifying this change

• This change is a trivial rework / code cleanup without any test coverage.

Documentation

• Does this pull request introduce a new feature? => no

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[StephanEwen]

Oh wow, the docs here are really outdated and wrong.

The correct way would be as follows:

POJOs are generally represented with a PojoTypeInfo and serialized with the PojoSerializer (using Kryo as a configurable fallback).

The exception is when the POJOs are actually Avro types (Avro Specific Records) or produced as "Avro Reflect Types". In that case the POJO's type would be represented by an AvroTypeInformation and use the AvroSerializer.

[StephanEwen]

Do you want to update the PR with this extra information?

[robstoll]

I'll will in the next days

[robstoll]

@StephanEwen I tried to integrate your information into my PR

[robstoll]

@zentol fixed https and reverted KryoSerializer