Document output of `fork` task

[matthias-pichler]

What would you like to be added:

Currently the output of the fork task is undocumented. My understanding is that:

1. compete: false: an array of the outputs from each branch in order of branch definition
2. compete: true: the output of the first completed branch

Why is this needed:

Questions:

Alternatively one could argue that compete: true could also return an array with all but the winning branch being null. This would allow downstream tasks to know which branch won but seems rather cumbersome.

[github-actions]

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