add to doc: guidance for choosing polynomial (#213)

[onzerem]

Hi! Thank you for saying yes to my PR. Here's what I added, please let me know what you think. I tried to keep it brief. Thank you!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.10%. Comparing base (dd8bbb2) to head (9ee2344).
Report is 1 commits behind head on main.

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  Coverage   92.10%   92.10%           
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  Files          19       19           
  Lines        1863     1863           
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  Hits         1716     1716           
  Misses        147      147           

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[dlfivefifty]

I think it should be phrased more openly, i.e., they are useful for many other things.

Eg., as an author of https://epubs.siam.org/doi/abs/10.1137/120865458?casa_token=b7rSj9AOym4AAAAA:7VcJMj3ueNw2Hx-yKH1JsYFeSNPYeARhrzXR3XJ0v22uZ_jDAav14MQy0BRYp7Ka_VOws90 it would be odd for me to only say ultraspherical polynomials are "useful for spherical harmonics in higher dimensions"

[onzerem]

@dlfivefifty Hi, I made an updated pr with your recommendation to use more open language. Hopefully, I was able to fulfill that without being too wordy, please let me know what you think. Thanks!

I also agree that this could be a good complement #222.