doc: add the missing deprecated method 'tmpDir()'

[ghost]

'tmpDir()' is first introduced in v0.7.11 (https://github.com/nodejs/node/releases/tag/v0.7.11), and then changed from 'tmpDir()' to 'tmpdir()' since v0.9.9 (3fe6aba).
Considering this is a deprecated method, add it as usual in the doc.

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• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines

[lpinca]

Does it make sense to document it now that is deprecated? I mean it was not documented, why should we do it now?

[ghost]

@lpinca：So you mean this wasn't documented on purpose?

[lpinca]

I don't know but I think it's better to keep it undocumented.

[silverwind]

Fine with me to document it I suppose, but please move it below tmpdir so CTRL-F will find the correct function first.

[ghost]

@silverwind：Thanks! I've corrected that, please have a review.

[tniessen]

Thank you for the contribution, @ghost, and sorry about the long inactivity!

I don't think we should add new documentation for a previously undocumented, deprecated API. Since node 8, os.tmpDir() has a runtime deprecation, so we have actively discouraged people from using it for at least two years. Adding documentation now seems pointless, if not misleading.