Some clarification of what functions do

[Tobychev]

After struggling to understand what these functions do, I reworked the doc strings to be a bit more clear, added missing return value annotations.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (4b617cc) 95.34% compared to head (fdd61c2) 95.35%.
Report is 4 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #266   +/-   ##
=======================================
  Coverage   95.34%   95.35%           
=======================================
  Files          60       60           
  Lines        3096     3097    +1     
=======================================
+ Hits         2952     2953    +1     
  Misses        144      144           

Files	Coverage Δ
pyirf/binning.py	96.66% <ø> (ø)
pyirf/cuts.py	95.83% <ø> (ø)

... and 1 file with indirect coverage changes

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