Docs: Add Vinyl.isVinyl() documentation

docs/api/vinyl-isvinyl.md

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+<!-- front-matter
+id: api-vinyl-isvinyl
+title: Vinyl.isVinyl()
+hide_title: true
+sidebar_label: Vinyl.isVinyl()
+-->
+
+# Vinyl.isVinyl()
+
+Determines if an object is a Vinyl instance. Use this method instead of `instanceof`.
+
+**Note**: This method uses an internal property that some older versions of Vinyl didn't expose resulting in a false negative if using an outdated version.
+
+## Usage
+
+```js
+const Vinyl = require('vinyl');
+
+const file = new Vinyl();
+const notAFile = {};
+
+Vinyl.isVinyl(file) === true;
+Vinyl.isVinyl(notAFile) === false;
+```
+
+## Signature
+
+```js
+Vinyl.isVinyl(file);
+```
+
+### Parameters
+
+| parameter | type | note |
+|:--------------:|:------:|-------|
+| file | object | The object to check. |
+
+### Returns
+
+True if the `file` object is a Vinyl instance.
+