doc: clarify how fs.Dirent file types are determined

The file type of a directory entry returned by fs.readdir() or
fs.opendir() is the type reported by the operating system's directory
listing. Node.js falls back to lstat() only when the reported type is
unknown, and some file systems may report a type that differs from what
lstat() would return. Document this in the fs.Dirent class introduction,
with fs.lstat() as the accurate alternative.

Fixes: #30646
Signed-off-by: Julian Soreavis <julian.soreavis@gmail.com>

Author: soreavis

doc/api/fs.md

@@ -7336,6 +7336,13 @@ Additionally, when [`fs.readdir()`][] or [`fs.readdirSync()`][] is called with
 the `withFileTypes` option set to `true`, the resulting array is filled with
 {fs.Dirent} objects, rather than strings or {Buffer}s.
 
+When a directory is read, such as with [`fs.readdir()`][] or
+[`fs.opendir()`][], the file type of each entry is the type reported by the
+operating system and may depend on the file system; for example, some file
+systems may report a type that differs from what [`fs.lstat()`][] returns.
+Node.js calls [`fs.lstat()`][] on such an entry only when the reported type
+is unknown. Use [`fs.lstat()`][] when an accurate file type is required.
+
 #### `dirent.isBlockDevice()`
 
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