Add a sentence clarifying that L.bind is not a polyfill for ES5 bind.

Fixes #2988.

Adds a sentence to the documentation: 
> Not a polyfill for ES 5 <code>bind</code> (compare <code>L.bind</code> to the <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Function/bind#Polyfill">MDN-recommended polyfill</a> for <code>Function.prototype.bind</code>)