No documentation about Data Length Extension

There's currently no documentation about Data Length Extension.

It is silly to list a feature on the front page of documentation that isn't actually documented in any normal way.

No file, test, or documentation entry point is identified. Start by locating the front-page reference to Data Length Extension and the existing documentation structure, then document the feature in the appropriate normal documentation location and ensure the front-page listing points to usable material.

Written by the indexing model from the issue text.