* docs: fix typo 'preformat' -> 'performant'
* docs: clarify that proxy returns _only_ enabled toggles
This is based on feedback we got that said:
> Right now biggest disappointment is that docs doesn't match an exact
> JS proxy libraries and their usage. For example, we couldn't make
> feature flag return enabled: false, instead it just hides/not return
> the flag at all.
