doc: document --no-warnings=<cat1>,<cat2>

Refs #30810.

Node allows suppression of e.g. experimental warnings with --no-warnings=ExperimentalWarning but that syntax is completely undocumented. Even the warning categories aren't documented anywhere, as far as I can tell.

The way node parses flags means its inversion --warnings=ExperimentalWarning possibly also does something meaningful but I haven't checked.