docs(schema): add examples to schema functions

Re: #7525

lib/schema.js

@@ -784,6 +784,14 @@ Schema.prototype.eachPath = function(fn) {
 /**
  * Returns an Array of path strings that are required by this schema.
  *
+ * ####Example:
+ *     const s = new Schema({
+ *       name: { type: String, required: true },
+ *       age: { type: String, required: true },
+ *       notes: String
+ *     });
+ *     s.requiredPaths(); // [ 'age', 'name' ]
+ *
  * @api public
  * @param {Boolean} invalidate refresh the cache
  * @return {Array}
@@ -828,6 +836,14 @@ Schema.prototype.indexedPaths = function indexedPaths() {
  *
  * Given a path, returns whether it is a real, virtual, nested, or ad-hoc/undefined path.
  *
+ * ####Example:
+ *     const s = new Schema({ name: String, nested: { foo: String } });
+ *     s.virtual('foo').get(() => 42);
+ *     s.pathType('name'); // "real"
+ *     s.pathType('nested'); // "nested"
+ *     s.pathType('foo'); // "virtual"
+ *     s.pathType('fail'); // "adhocOrUndefined"
+ *
  * @param {String} path
  * @return {String}
  * @api public
@@ -1059,6 +1075,14 @@ function getPositionalPath(self, path) {
 /**
  * Adds a method call to the queue.
  *
+ * ####Example:
+ *
+ *     schema.methods.print = function() { console.log(this); };
+ *     schema.queue('print', []); // Print the doc every one is instantiated
+ *
+ *     const Model = mongoose.model('Test', schema);
+ *     new Model({ name: 'test' }); // Prints '{"_id": ..., "name": "test" }'
+ *
  * @param {String} name name of the document method to call later
  * @param {Array} args arguments to pass to the method
  * @api public
@@ -1168,6 +1192,12 @@ Schema.prototype.post = function(name) {
 /**
  * Registers a plugin for this schema.
  *
+ * ####Example:
+ *
+ *     const s = new Schema({ name: String });
+ *     s.plugin(schema => console.log(schema.path('name').path));
+ *     mongoose.model('Test', schema); // Prints 'name'
+ *
  * @param {Function} plugin callback
  * @param {Object} [opts]
  * @see plugins
@@ -1510,6 +1540,13 @@ Schema.prototype.virtualpath = function(name) {
 /**
  * Removes the given `path` (or [`paths`]).
  *
+ * ####Example:
+ *
+ *     const schema = new Schema({ name: String, age: Number });
+ *     schema.remove('name');
+ *     schema.path('name'); // Undefined
+ *     schema.path('age'); // SchemaNumber { ... }
+ *
  * @param {String|Array} path
  * @return {Schema} the Schema instance
  * @api public