docs: add styled rules description supplement

[GyeongtaeGim]

Summary

description is for xstyled, not styled.

const Box = styled.div({
  margin: 2, // '2px' (not transformed by xstyled)
  marginTop: '2', // '8px' (transformed by styled)
})

and description of value conversion by theme is insufficient.

Test plan

Just document update.