Add a hint to reflect in the documentation of parallel

[PierreFritsch]

How to continue parallel processing in case of a failing task seems to be a frequently asked question. It was mentioned in several issues:

• async parallel error handling #334
• Looking for a way to not exit on async.parallel on error. #675
• async.parallel - allowing all functions to finish even if error is thrown #798
• "Reflect" behavior for collecting errors/results and continuing. #942

Adding a hint to the documentation might help users of the library to find out about the answer to this question.

[megawac]

Thanks! Yeah very common question thanks for the documenting it!