docs(python): Fix formula in ewm_mean_by

[MarcoGorelli]

closes #18377

the ewm_mean part can be improved too...will address this a bit later today

This does a few things:

• corrects the formula for ewm_mean_by to include 1- for \alpha
• makes the ewm_mean variable for half-life consistent with the ewm_mean_by one (and also with https://quant.stackexchange.com/a/46197)
• adds verbs at the beginning of these docstrings

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.85%. Comparing base (4dc90a9) to head (226bc37).
Report is 14 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #18506      +/-   ##
==========================================
- Coverage   79.87%   79.85%   -0.02%     
==========================================
  Files        1501     1501              
  Lines      202032   201813     -219     
  Branches     2868     2868              
==========================================
- Hits       161370   161160     -210     
+ Misses      40115    40106       -9     
  Partials      547      547              

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.