Why isn't `fd` option of `fs.createWriteStream` documented?

[natevw]

Although just barely (see #7707), the fd option to fs.createReadStream is publicly documented. However for fs.createWriteStream no mention is made, although I see in the code it is supported.

Is this feature supposed to be publicly used only with readStream, or encouraged with both, or publicly supported with neither?

[natevw]

FWIW, it looks like the following interesting facts are true about passing an fd to fs.createWriteStream:

• the 'open' event is not fired if you pass in a file descriptor
• once 'finish' fires, fd will be closed, i.e. there is no support for an 'autoClose:false' option like fs.createReadStream
• …TBD…

DISCLAIMER — please don't take the above "documentation" as an official API commitment, I am just some yokel nosing around in someone's else's code to see how it might currently behave.

[chrisdickinson]

Addressed by #8001.