doc: fs docs should mention that `mode` is ignored when file exists

[bnoordhuis]

See subject.

On that topic, it's kind of odd that fs.createReadStream() accepts a mode argument. I guess it makes sense if you allow people to pass e.g. flags='w' but otherwise it's always a no-op.

[trevnorris]

@bnoordhuis PR's welcome. ;-P

But seriously, have it on my list.

[nelsonpecora]

Looks like it's mentioned in the fs.open section. Is that acceptable?

[jasnell]

Landed in 1b027ee