Clarify that an Er dipole at r=0 and m=±1 has circular polarization

[oskooi]

Adds a note to Tutorial/Extraction Efficiency of a Collection of Dipoles in a Disc based on the discussion in #2917 (comment):

Note that an $E_r$ dipole at $r = 0$ with $m = \pm 1$ has circular (*not* linear) polarization.

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All modified and coverable lines are covered by tests ✅

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@@            Coverage Diff             @@
##           master    #2949      +/-   ##
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- Coverage   73.81%   73.71%   -0.10%     
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  Files          18       18              
  Lines        5423     5449      +26     
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+ Hits         4003     4017      +14     
- Misses       1420     1432      +12