Clarify CoGroupByKey creates Iterable, not list.

[rainwoodman]

The docsting was strongly implying the contract was to create lists, which is not the intention.

GroupByKey returns a Iterable.

@robertwb @katsiapis @dmkozh

Please add a meaningful description for your change here

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ValidatesRunner compliance status (on master branch)

Lang	ULR	Dataflow	Flink	Samza	Spark	Twister2
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Java
Python	---					---
XLang						---

Examples testing status on various runners

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Java	---		---	---	---	---	---
Python	---	---	---	---	---	---	---
XLang	---	---	---	---	---	---	---

Post-Commit SDK/Transform Integration Tests Status (on master branch)

Go	Java	Python

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website	Whitespace	Typescript
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Portable	---			---	---	---

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[rainwoodman]

R: @robertwb

[codecov]

Codecov Report

Merging #16099 (e709d39) into master (88429b2) will increase coverage by 37.56%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master   #16099       +/-   ##
===========================================
+ Coverage   46.08%   83.65%   +37.56%     
===========================================
  Files         195      447      +252     
  Lines       19457    61692    +42235     
===========================================
+ Hits         8967    51607    +42640     
- Misses       9521    10085      +564     
+ Partials      969        0      -969     

Impacted Files	Coverage Δ
sdks/python/apache_beam/transforms/util.py	95.86% <ø> (ø)
sdks/go/pkg/beam/core/graph/bind.go
...dks/go/pkg/beam/core/runtime/harness/monitoring.go
sdks/go/pkg/beam/transforms/top/top.go
sdks/go/pkg/beam/util/gcsx/gcs.go
sdks/go/pkg/beam/core/runtime/exec/plan.go
sdks/go/pkg/beam/core/runtime/exec/window.go
sdks/go/pkg/beam/core/runtime/graphx/user.go
sdks/go/pkg/beam/core/runtime/coderx/varint.go
sdks/go/pkg/beam/transforms/stats/count.go
... and 633 more

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[TheNeuralBit]

Thank you!