docs(middleware): list validate as a potential query middleware

[vkarpov15]

Re: #12680

Summary

#12680 pointed out that we don't list 'validate' as a potential query middleware. This fixes the docs. Below is an example of using query middleware for validate()

'use strict';

const mongoose = require('mongoose');

const schema = new mongoose.Schema({ name: String });

schema.pre('validate', () => {
  console.log('Regular pre validate');
});
schema.pre('validate', { query: true, document: false }, () => {
  console.log('Pre query validate');
});

const Test = mongoose.model('Test', schema);

run().catch(err => console.log(err));

async function run() {
  await mongoose.connect('mongodb://localhost:27017/test');

  const doc = new Test();
  await doc.validate();

  console.log('----');
  await Test.updateOne({}, { name: 'bar' }).setOptions({ runValidators: true });
}

Examples