Update Basic Tutorial about missing @hydra.main return

[kychow]

Add info about lack of return value for @hydra.main due to ambiguity to the basics tutorial. See #769

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[omry]

Thanks @kychow, please open an issue for discussion before filing a PR.

[omry]

Thanks @kychow.

This doesn't really fit in the basic tutorial.
The basic tutorial is introducing Hydra to users, this is not something users following the tutorial should do.

I am considering creating a page describing @hydra.main in more details (similar to this which will get into details of this nature).