Tweak Elasticsearch howto docs

What does this Merge Request do?

Explains where the default ES version is defined, and how to use a different one.

Closes #823 (closed)

/cc @tnir, let me know if this is clear enough 🙂

Merge Request checklist

• [-] Tests added for new functionality. If not, please raise Issue to follow-up.
• This change is backward compatible. If not, please include steps to communicate to our users.
• Documentation added/updated, if needed.
• [-] gdk doctor test added, if needed.