docs: Clarify that the proxy does not expose disabled flags (#1094)
docs: Clarify that the proxy does not expose disabled flags (#1094)

* 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.
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