Nim: document `nim --nep1:on`

[timotheecour]

we should document the switch --nep1:on ; it may not be perfect but still useful; we can just document it in --fullhelp and specify it's still experimental; that'll make more people use it and make it easier to find/report bugs with this tool in the wild

learned about it here: https://github.com/nim-lang/Nim/pull/9510#issuecomment-434180531

[Araq]

Before I document it I want to change it to --styleCheck:off|hints|error.

[krux02]

@Araq yes, a much better name.