[MINOR][CORE] Fix incorrect documentation of WritableConverter

## What changes were proposed in this pull request?

`WritableConverter` and `WritableFactory` work in opposite directions. But both of them are documented with same description:

> A class encapsulating how to convert some type T to Writable. It stores both the Writable class corresponding to T (e.g. IntWritable for Int) and a function for doing the conversion.

This error is a result of commit 2604939f. As a note, `WritableFactory` was added from commit d37978d8, which resolves [SPARK-4795](https://issues.apache.org/jira/browse/SPARK-4795) with the correct description.

This PR fix the documentation of `WritableConverter`, along with some improvements on type description.

## How was this patch tested?

`build/mvn clean checkstyle:checkstyle`

Author: Lee Dongjin <dongjin@apache.org>

Closes #16830 from dongjinleekr/feature/fix-writableconverter-doc.