Improve degree_counts doc

[maximelucas]

Fix #190

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (3435409) 90.35% compared to head (13e283c) 90.35%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #329   +/-   ##
=======================================
  Coverage   90.35%   90.35%           
=======================================
  Files          41       41           
  Lines        2954     2954           
=======================================
  Hits         2669     2669           
  Misses        285      285           

Impacted Files	Coverage Δ
xgi/classes/function.py	100.00% <ø> (ø)

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