Description or memo with helpers

Helpers have come a long way since their introduction and recently I started moving some groups, timers etc which used to be in other places to helpers. The list of helpers is growing fast.

But now I notice that I miss a way to describe what that/where that helper is used. What it does.

Seem to me that with the growth of # of helpers and there more frequent use a description or memo possibility could be in order.

I was using comments all over the place in configuration.yaml. With config moving to the UI, it would be nice to have this on any entity. This FR is similar to others for custom user-defined and persistent attributes on entities.