Docs: Add Vinyl.isCustomProp() documentation

docs/api/vinyl-iscustomprop.md

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+<!-- front-matter
+id: api-vinyl-iscustomprop
+title: Vinyl.isCustomProp()
+hide_title: true
+sidebar_label: Vinyl.isCustomProp()
+-->
+
+# Vinyl.isCustomProp()
+
+Determines if a property is internally managed by Vinyl. Used by Vinyl when setting values inside the constructor or when copying properties in the `clone()` instance method.
+
+This method is useful when extending the Vinyl class. Detailed in [Extending Vinyl][extending-vinyl-section] below.
+
+## Usage
+
+```js
+const Vinyl = require('vinyl');
+
+Vinyl.isCustomProp('sourceMap') === true;
+Vinyl.isCustomProp('path') === false;
+```
+
+## Signature
+
+```js
+Vinyl.isCustomProp(property)
+```
+
+### Parameters
+
+| parameter | type | note |
+|:--------------:|:------:|-------|
+| property | string | The property name to check. |
+
+### Returns
+
+True if the property is not internally managed.
+
+## Extending Vinyl
+
+When custom properties are managed internally, the static `isCustomProp` method must be extended and return false when one of the custom properties is queried.
+
+```js
+const Vinyl = require('vinyl');
+
+const builtInProps = ['foo', '_foo'];
+
+class SuperFile extends Vinyl {
+  constructor(options) {
+    super(options);
+    this._foo = 'example internal read-only value';
+  }
+
+  get foo() {
+    return this._foo;
+  }
+
+  static isCustomProp(name) {
+    return super.isCustomProp(name) && builtInProps.indexOf(name) === -1;
+  }
+}
+```
+
+In the example above, `foo` and `_foo` will not be assigned to the new object when cloning or passed in `options` to `new SuperFile(options)`.
+
+If your custom properties or logic require special handling during cloning, override the `clone` method while extending Vinyl.
+
+[extending-vinyl-section]: #extending-vinyl