doc: specify how to detect EOF

Specify how to detect end-of-file when using `fs.read()` and
`filehandle.read()`.

PR-URL: #35445
Fixes: #35363
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Zeyu Yang <himself65@outlook.com>
Reviewed-By: Andrey Pechkurov <apechkurov@gmail.com>

doc/api/fs.md

@@ -2909,6 +2909,9 @@ If `position` is an integer, the file position will remain unchanged.
 
 The callback is given the three arguments, `(err, bytesRead, buffer)`.
 
+If the file is not modified concurrently, the end-of-file is reached when the
+number of bytes read is zero.
+
 If this method is invoked as its [`util.promisify()`][]ed version, it returns
 a `Promise` for an `Object` with `bytesRead` and `buffer` properties.
 
@@ -4657,6 +4660,9 @@ Following successful read, the `Promise` is resolved with an object with a
 `bytesRead` property specifying the number of bytes read, and a `buffer`
 property that is a reference to the passed in `buffer` argument.
 
+If the file is not modified concurrently, the end-of-file is reached when the
+number of bytes read is zero.
+
 #### `filehandle.read(options)`
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