doc: improve fsPromises docs

[benjamingr]

Improe the filesystem promises docs:

• Clarify promise terminology (promises are never fulfilled without a value - just like functions in JS).
• Add some links and parameter descriptions.

I wanted to put all the changes here - but this PR got pretty big pretty fast so I figured I'd split the changes into multiple smaller PRs to make review reasonable and reduce the amount of potential conflicts.

CC @RaisinTen

[aduh95]

Linter is failing.

[benjamingr]

At some day - I will figure out a workflow that will let me deal with the fact Node is the one project I often force-push to :D and I'll stop forgetting to check I did not get the "Updates were rejected" message.

[RaisinTen]

Suggested change

	If `position` is `null`, data will be read from the current file position,
	If `position` is `null` or `-1`, data will be read from the current file position,

[jasnell]

@benjamingr ... could these changes be bundled in with this PR instead to prevent conflicts? #37170

[benjamingr]

@jasnell sure.

I'd also like to apologize for about landing the resolved->fulfilled changes I would have waited with those as well if I would have seen your PR.

It would be super useful to have automation to tell you if changes you are proposing conflict with other open PRs.

[benjamingr]

@jasnell Would you prefer it if I pushed the changes to your branch or wait for it to land before re-opening this PR?

[jasnell]

Feel free to push to my branch if you'd like.

[jasnell]

And no worries about the conflicts. We can get that rebased pretty easily