docs(python): Improve docstrings for ewm_* and rolling_* methods

[MarcoGorelli]

A few minor things:

• for rolling, - 1i (1 index count) is already documented, I don't an extra paragraph showing it in bold is warranted
• for rolling_*, the default is now closed='right', so I think that's what the example should show. I don't think the explanation of closed='left' adds much
• for ewm_*, note that adjust=True is the default (it's quite a long docstring, easier to see it there than to scroll up to the signature)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.79%. Comparing base (849953b) to head (99f0f5b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #14667      +/-   ##
==========================================
- Coverage   80.79%   80.79%   -0.01%     
==========================================
  Files        1326     1326              
  Lines      173249   173249              
  Branches     2456     2456              
==========================================
- Hits       139976   139970       -6     
- Misses      32800    32806       +6     
  Partials      473      473              

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

[stinodego]

Looks good to me, thanks!