doc: add missing return types in fs.md

[chasetonco]

Three entries in the fs documentation described their return value only in prose, or not at all, so the documentation tooling could not parse a return type and fell back to void:

• filehandle[Symbol.asyncDispose]() and dir[Symbol.asyncDispose]() both return a promise. This matches the existing Returns: {Promise} annotations on other async dispose methods, such as heapProfileHandle[Symbol.asyncDispose]() in v8.md.
• new fs.Utf8Stream([options]) is a constructor and returns an instance of the class, annotated the same way as the fs.ReadStream and fs.WriteStream constructors in this file.

No prose was changed; only the structured Returns: lines were added.

Verified by calling each function on a local build and by rendering fs.md locally, which now shows Promise, Promise and fs.Utf8Stream instead of void.

Refs: nodejs/doc-kit#953

[nodejs-github-bot]

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----------------------------------- PR info ------------------------------------
Title      doc: add missing return types in fs.md (#65307)
   ⚠  Could not retrieve the email or name of the PR author's from user's GitHub profile!
Branch     chasetonco:doc-fs-return-types -> nodejs:main
Labels     doc, fs, author ready, commit-queue
Commits    1
 - doc: add missing return types in fs.md
Committers 1
 - Chxxeton <43923165+Chxxeton@users.noreply.github.com>
PR-URL: https://github.com/nodejs/node/pull/65307
Refs: https://github.com/nodejs/doc-kit/issues/953
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Aviv Keller <me@aviv.sh>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
------------------------------ Generated metadata ------------------------------
PR-URL: https://github.com/nodejs/node/pull/65307
Refs: https://github.com/nodejs/doc-kit/issues/953
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Aviv Keller <me@aviv.sh>
Reviewed-By: Ulises Gascón <ulisesgascongonzalez@gmail.com>
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   ℹ  This PR was created on Sat, 15 Aug 2026 14:13:12 GMT
   ✔  Approvals: 4
   ✔  - James M Snell (@jasnell) (TSC): https://github.com/nodejs/node/pull/65307#pullrequestreview-4944187613
   ✔  - Luigi Pinca (@lpinca): https://github.com/nodejs/node/pull/65307#pullrequestreview-4944240692
   ✔  - Aviv Keller (@avivkeller): https://github.com/nodejs/node/pull/65307#pullrequestreview-4946336401
   ✔  - Ulises Gascón (@UlisesGascon): https://github.com/nodejs/node/pull/65307#pullrequestreview-4946351571
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[nodejs-github-bot]

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