Document textTransform support
~**Proposal/Feature-request:**~

~Might be worth supporting the `textTransform` CSS property. This is supported by React Native now: https://reactnative.dev/docs/text-style-props#texttransform~

~`capitalize` isn't supported by Sketch though, so maybe this should be a transformation applied to the text string children manually? Or added to the docs with a strikethrough for `capitalize`.~

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Sorry, forgot to check existing issues 😄, just checked the source and it already exists. 

`textTransform` is missing from the docs though, so will add it soon, makes sense to do it together with cleaning up the docs a bit more though so the PR isn't too atomic.