Fill missing javadocs

[IvanKiral]

Motivation

Fill up some missing javadocs mostly in delivery SDKs

Proposed solution

Check and add javadocs. The few which are missing are: Asset.java, DeliveryClient.java, RichTextElement.java, RichTextElementConverter.java, StronglyTpyedContentItemConverter.java.

Additional context

Add any other context, screenshots, or reference links about the feature request here.

[iremugurlu]

Hey, I'm interested. Can you assign this one to me? Thanks.