doc: add missing return types in fs.md

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

* `filehandle[Symbol.asyncDispose]()` and
  `dir[Symbol.asyncDispose]()` both return a promise, matching
  the existing `Returns: {Promise}` annotations on other async
  dispose methods.
* `new fs.Utf8Stream([options])` is a constructor and returns
  an instance of the class.

Verified at runtime and by rendering the page locally with
doc-kit.

Refs: nodejs/doc-kit#953
Signed-off-by: Chxxeton <43923165+Chxxeton@users.noreply.github.com>
PR-URL: #65307
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>

Author: chasetonco

doc/api/fs.md

@@ -1184,6 +1184,8 @@ changes:
    description: No longer experimental.
 -->
 
+* Returns: {Promise}
+
 Calls `filehandle.close()` and returns a promise that fulfills when the
 filehandle is closed.
 
@@ -7317,6 +7319,8 @@ changes:
    description: No longer experimental.
 -->
 
+* Returns: {Promise}
+
 Calls `dir.close()` if the directory handle is open, and returns a promise that
 fulfills when disposal is complete.