random lz4f clarifications

random lz4f clarifications

the initial intention was to update lz4f ring buffer strategy,
but lz4f doesn't use ring buffer.
Instead, it uses the destination buffer as much as possible,
and merely copies just what's required to preserve history
into its own buffer, at the end.
Pretty efficient.

This patch just clarifies a few comments and add some assert().
It's built on top of #528.

It also updates doc.