Add clarification on Filter transform's input function to pydoc.

[tomstepp]

It was difficult to understand how to implement a function to pass to the Filter transform. I added a useful comment that was found in "https://beam.apache.org/documentation/transforms/python/elementwise/filter/".

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Codecov Report

Merging #17704 (63ce79d) into master (6774b74) will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #17704      +/-   ##
==========================================
- Coverage   74.00%   73.99%   -0.02%     
==========================================
  Files         695      695              
  Lines       91798    91798              
==========================================
- Hits        67938    67922      -16     
- Misses      22612    22628      +16     
  Partials     1248     1248              

Flag	Coverage Δ
python	83.73% <ø> (-0.03%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
sdks/python/apache_beam/transforms/core.py	92.30% <ø> (ø)
sdks/python/apache_beam/internal/gcp/auth.py	73.33% <0.00%> (-5.34%)	⬇️
.../python/apache_beam/testing/test_stream_service.py	88.09% <0.00%> (-4.77%)	⬇️
...che_beam/runners/interactive/interactive_runner.py	89.20% <0.00%> (-1.44%)	⬇️
...eam/runners/portability/fn_api_runner/execution.py	92.44% <0.00%> (-0.65%)	⬇️
sdks/python/apache_beam/transforms/combiners.py	93.05% <0.00%> (-0.39%)	⬇️
...hon/apache_beam/runners/worker/bundle_processor.py	93.43% <0.00%> (-0.25%)	⬇️

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[tomstepp]

R: @tvalentyn

[tomstepp]

Valentyn - I'm not sure who to add as reviewer, can you please take a look or help me find correct reviewer?

[tvalentyn]

Thank you!