fix(index.d.ts): fix Document#populate() type

[thiagokisaki]

Summary
According to mongoose docs, Document#populate() receives, as the first argument, the path to populate or an options object or an array of paths and/or options objects.
However, in the following code, there is a TypeScript error:

import { Schema, model } from 'mongoose';

const personSchema = new Schema({
  name: String,
  stories: [{ type: Schema.Types.ObjectId, ref: 'Story' }]
});

const storySchema = new Schema({
  title: String,
  author: { type: Schema.Types.ObjectId, ref: 'Person' },
  fans: [{ type: Schema.Types.ObjectId, ref: 'Person' }]
});

const Person = model('Person', personSchema);
const Story = model('Story', storySchema);

(async () => {
  const story = await Story.findOne().orFail(new Error('Story not found'));
  
  await story.populate(['author', { path: 'fans' }]); // This line is causing the error
})();

No overload matches this call.
  Overload 1 of 6, '(path: string | string[]): Promise<Document<any, any, unknown>>', gave the following error.
    Type '{ path: string; }' is not assignable to type 'string'.
  Overload 2 of 6, '(opts: PopulateOptions | PopulateOptions[]): Promise<Document<any, any, unknown>>', gave the following error.
    Type 'string' is not assignable to type 'PopulateOptions'.ts(2769)

This PR solves the problem, allowing us to provide an array mixing paths and option objects.

Examples
Now, all of these lines are valid:

  await story.populate('author');
  await story.populate({ path: 'fans' });
  await story.populate(['author']);
  await story.populate([{ path: 'fans' }]);
  await story.populate(['author', { path: 'fans' }]);

[AbdelrahmanHafez]

Looks great, would appreciate some tests proving that this works.

[vkarpov15]

Great work, thanks 👍