Missing documentation on P300 paradigm

[toncho11]

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When checking:

https://moabb.neurotechx.com/docs/generated/moabb.paradigms.P300.html

It looks like the parameters for the P300 paradigm are not listed. There should be "resample", "fmin", "fmax", right? In the current state it is not clear if by default the signal is filtered("fmin", "fmax") when the P300 paradigm is used and this is confusing.

[bruAristimunha]

Related with #467