Make `optuna.samplers.nsgaii` part more informative

[nzw0301]

What is an issue?

We make optuna.samplers.nsgaii (see the screenshot below or documentation) more informative to guess what APIs are explained on the linked page.

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[HideakiImamura]

I think it is sufficient to add a note section above the optuna.samplers.nsgaii part like that above the following optuna.visdualization.matplotlib section. https://optuna.readthedocs.io/en/latest/reference/visualization/index.html