[doc] `Check` mode does not supress fatal errors

[lolbinarycat]

I was trying to write my own combinator and was getting very confused as to why there were two different error types, and as to why Check didn't seem to be suppressing every error, despite the docs saying that it does.

[Geal]

Check in general is used to verified that an output or error was produced, to continue parsing after that. The fatal errors are there to indicate that parsing should stop entirely, so the error passes through

[lolbinarycat]

yes, but that's not what the docs say.