doc: improve escaping documentation

[tylerablackham]

Addresses the following issue: handlebars-lang/handlebars.js#2094

It seems like there has been some confusion on how the raw blocks actually work. I had to play with it a while to understand how it worked. My changes are to improve the documentation and example of how to use the {{{{ mustache braces to create a raw block.

[tylerablackham]

@jaylinski The documentation before made it seem like raw was supposed to be a built-in helper, but with my experiments, I wasn't able to use a built in helper called raw. I had to create the helper myself. In this PR, I assumed that raw is a custom helper that the user has to write themselves. Is there any merit for creating a built-in raw helper? If so I can create an issue for it.