message: docs(isDocument): update for current implementation
diff: @@ -8,7 +8,7 @@ title: 'isDocument & isDocumentArray'
  **Typings:**
  ```ts
  -function isDocument<T, S extends RefType>(doc: Ref<T, S>): doc is DocumentType<T>
  +function isDocument<T, S extends RefType>(doc: Ref<T, S> | null | undefined): doc is DocumentType<T>
  ```
  **Parameters:**
  @@ -29,7 +29,7 @@ class Cat {
  // this example could be smaller, but for demonstation purposes this is a longer version
  public hasPartner(): boolean {
  if (isDocument(this.partner)) {
  - // "this.partner" now has the type of "Cat"
  + // "this.partner" now has the type of "DocumentType<Cat>"
  return true;
  } else {
  return false;
  @@ -39,8 +39,7 @@ class Cat {
  ```
  :::tip
  -Alternatively, since mongoose 6.4.0, [`$assertPopulated`](https://mongoosejs.com/docs/api/document.html#document_Document-undefined) can also be used.
  -Note: the documentation link is currently broken and may change, see [mongoose #11957](https://github.com/Automattic/mongoose/issues/11957).
  +Alternatively, since mongoose 6.4.0, [`$assertPopulated`](https://mongoosejs.com/docs/api/document.html#document_Document-$assertPopulated) can also be used, but that function does not act as a type-guard only at runtime to actually check if a full path is populated.
  :::
  ## isDocumentArray
  @@ -91,7 +90,7 @@ class Cat {
  // this example could be smaller, but for demonstation purposes this is a longer version
  public areAllKittensExisting(): boolean {
  if (isDocumentArray(this.kittens)) {
  - // "this.kittens" now has the type of "Cat"
  + // "this.kittens" now has the type of "DocumentType<Cat>[]"
  return true;
  } else {
  return false;
  