events, doc: add type checking in defaultMaxListeners getter

Since EventEmitter.defaultMaxListeners and emitter.setMaxListeners(n) are both public APIs listed in the document, so it is reasonable to check EventEmitter.defaultMaxListeners getter's input like what emitter.setMaxListeners(n) does to avoid potential confusing results or error messages caused by some userland mistaken input, like:

'use strict'
const E = require('events')

E.defaultMaxListeners = new Proxy({}, { get: () => {} })
let e = new E()
e.addListener('1', function () {})
e.addListener('1', function () {})
// output:
// events.js:260
//      if (m && m > 0 && existing.length > m) {
//                 ^
//
// TypeError: Cannot convert object to primitive value
//     at _addListener (events.js:260:18)
//     at EventEmitter.addListener (events.js:279:10)
//     at Object.<anonymous> (/Users/caiwei/workspace/benchmarks/index.js:7:3)
//     at Module._compile (module.js:571:32)
//     at Object.Module._extensions..js (module.js:580:10)
//     at Module.load (module.js:488:32)
//     at tryModuleLoad (module.js:447:12)
//     at Function.Module._load (module.js:439:3)
//     at Module.runMain (module.js:605:10)
//     at run (bootstrap_node.js:425:7)

Checklist

• [-] make -j4 test (UNIX), or vcbuild test (Windows) passes
• [-] tests and/or benchmarks are included
• [-] documentation is changed or added
• [-] commit message follows commit guidelines

Affected core subsystem(s)

events, doc