Add method to get all populated docs on a document

[vkarpov15]

Re: #8115 . doc.getPopulatedDocs() should return an array of all populated documents associated with this doc.

const doc = await Story.findOne({ title: 'Casino Royale' }).populate('fans');
console.log(doc.getPopulatedDocs()); // List of all `fans` documents

[AlexRMU]

$getPopulatedDocs is in the code and types, but not in the documentation