[Popper] Fix anchorEl prop types

[dan8f]

• I have followed (at least) the PR section of the contributing guide.

@oliviertassinari, quick PR to discuss your proposal #15922

Closes #15922

[eps1lon]

I think we can remove any notion of Element now. Typing anchorEl structurally with the ReferenceObject seems sufficient. Element is just an example implementation of the ReferenceObject interface.

[oliviertassinari]

@eps1lon I haven't thought about this approach. Yeah, that sounds fair 👍.

[mui-pr-bot]

No bundle size changes comparing 64dab48...7c4f7ae

Generated by 🚫 dangerJS against 7c4f7ae

[eps1lon]

Would only mention AnchorElement BikeshedMyNamePlease¹ and link to the API docs describing that interface.

¹We can't use AnchorElement because that points to an instance of <a />. Maybe ReferenceObject is better after all.

[oliviertassinari]

@eps1lon Maybe it's good enough as it is now?

[eps1lon]

@eps1lon Maybe it's good enough as it is now?

I mean you raised the concern so you should answer this. I was ok with ReferenceObject but offered another solution after concerns were raised.

[oliviertassinari]

@eps1lon OK, after more digging, I'm fine with keeping ReferenceObject too :).

[oliviertassinari]

I have updated the wording. Do you guys see something left to address?

[oliviertassinari]

@dan8f It's a great first pull request on Material-UI 👌🏻. Thank you for working on it!