Document sequential vs parallel tuning recommended practices

[luisquintanilla]

Add to Parallel tuning sections.

https://microsoft.github.io/FLAML/docs/Use-Cases/Task-Oriented-AutoML#parallel-tuning

https://microsoft.github.io/FLAML/docs/Use-Cases/Tune-User-Defined-Function#parallel-tuning

[liususan091219]

Add one concrete example of NLP of selecting parallel vs sequential.

[qingyun-wu]

Add to Parallel tuning sections.

https://microsoft.github.io/FLAML/docs/Use-Cases/Task-Oriented-AutoML#parallel-tuning

https://microsoft.github.io/FLAML/docs/Use-Cases/Tune-User-Defined-Function#parallel-tuning

Documentation is added: https://microsoft.github.io/FLAML/docs/Use-Cases/Task-Oriented-AutoML#guidelines-on-parallel-vs-sequential-tuning

[qingyun-wu]

Hi @liususan091219, thank you for offering to help. Are you going to add the example you mentioned? If so, when do you plan to add it?

Thank you!