Correction (and clarification) to EWMA doc

[jamesoliverh]

Correct adjust=True to adjust=False in line 889 of the documentation (discussing EWMA).
Also propose minor clarifications in demonstration that two variants (adjust=False and adjust=True) are equivalent for infinite series (lines 868 and 887-893).

Closes #24814

[codecov]

Codecov Report

Merging #26422 into master will decrease coverage by <.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26422      +/-   ##
==========================================
- Coverage   91.69%   91.69%   -0.01%     
==========================================
  Files         174      174              
  Lines       50741    50741              
==========================================
- Hits        46529    46525       -4     
- Misses       4212     4216       +4

Flag	Coverage Δ
#multiple	90.19% <ø> (ø)	⬆️
#single	41.16% <ø> (-0.18%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97.02% <0%> (-0.12%)	⬇️

---

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[codecov]

Codecov Report

Merging #26422 into master will increase coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #26422      +/-   ##
==========================================
+ Coverage   91.69%   91.72%   +0.02%     
==========================================
  Files         174      174              
  Lines       50741    50745       +4     
==========================================
+ Hits        46529    46547      +18     
+ Misses       4212     4198      -14

Flag	Coverage Δ
#multiple	90.23% <ø> (+0.03%)	⬆️
#single	41.69% <ø> (+0.36%)	⬆️

Impacted Files	Coverage Δ
pandas/io/gbq.py	78.94% <0%> (-10.53%)	⬇️
pandas/core/frame.py	97.02% <0%> (-0.12%)	⬇️
pandas/util/testing.py	90.6% <0%> (-0.11%)	⬇️
pandas/core/base.py	97.99% <0%> (-0.01%)	⬇️
pandas/tseries/offsets.py	96.69% <0%> (-0.01%)	⬇️
pandas/core/reshape/reshape.py	99.56% <0%> (ø)	⬆️
pandas/io/pytables.py	90.22% <0%> (ø)	⬆️
pandas/core/window.py	96.41% <0%> (ø)	⬆️
pandas/core/reshape/merge.py	94.47% <0%> (ø)	⬆️
pandas/core/internals/blocks.py	94.08% <0%> (ø)	⬆️
... and 3 more

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[gfyoung]

cc @jreback

[jreback]

thanks @jamesoliverh