Document ScioIO methods, fixes #3937

[kellen]

No description provided.

[codecov]

Codecov Report

Merging #4689 (eb0eeb5) into main (bdb0922) will decrease coverage by 7.69%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #4689      +/-   ##
==========================================
- Coverage   68.32%   60.64%   -7.69%     
==========================================
  Files         269      286      +17     
  Lines        8228    10456    +2228     
  Branches      673      669       -4     
==========================================
+ Hits         5622     6341     +719     
- Misses       2606     4115    +1509     

Impacted Files	Coverage Δ
...re/src/main/scala/com/spotify/scio/io/ScioIO.scala	55.00% <ø> (ø)
...ala/com/spotify/scio/testing/CoderAssertions.scala	80.64% <0.00%> (-19.36%)	⬇️
...spotify/scio/schemas/instances/JavaInstances.scala	48.14% <0.00%> (-10.95%)	⬇️
...la/com/spotify/scio/coders/CoderMaterializer.scala	92.85% <0.00%> (-7.15%)	⬇️
...ain/scala/com/spotify/scio/coders/BeamCoders.scala	76.19% <0.00%> (-1.59%)	⬇️
.../main/scala/com/spotify/scio/pubsub/PubsubIO.scala	10.38% <0.00%> (-0.88%)	⬇️
...src/main/scala/com/spotify/scio/coders/Coder.scala	85.39% <0.00%> (-0.77%)	⬇️
...la/com/spotify/scio/bigquery/client/BigQuery.scala	22.03% <0.00%> (-0.42%)	⬇️
...y/scio/extra/sorter/syntax/SCollectionSyntax.scala	94.44% <0.00%> (-0.30%)	⬇️
...n/scala/com/spotify/scio/coders/DerivedCoder.scala	97.43% <0.00%> (ø)
... and 21 more

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