doc: updated fs #5862

[ToPal]

See issue #5862.
Removed irrelevant data about fs.readFile in docs for version 4.x

[mscdex]

I'm not sure if it was intentional that file descriptors are not working with fs.readFile(), but assuming it was intentional, there are a couple of other comments at the end of the fs.readFile() section there that need to be removed also.

[jasnell]

@ToPal ... thanks for this. Since this is for v4.x, the PR actually needs to be opened against the v4.x-staging branch instead of directly against v4.x. We land all changes into the staging branch first and only pull things into v4.x when there's a release.

[ToPal]

Thanks, @mscdex! I fixed it.

[ToPal]

@jasnell Thank you for information! I've created a new pull request for branch v4.x-staging.