Documentation: Add additional information about the canvas package

[DanKaplanSES]

• I removed the information about the canvas package being a "peer dependency" because, as far as I could tell, this was an implementation detail. If I'm right about that assumption, it could confuse users who are unfamiliar with the concept.
• I reiterated information about the relationship between the canvas package, the <img> element, and the ResourceLoader. I believe this redundancy is warranted because users could spend a lot of time troubleshooting if they miss this detail.

[DanKaplanSES]

Not sure how to rerun this but the failure doesn't seem related to my change? LMK if I'm wrong about that.

[domenic]

• I removed the information about the canvas package being a "peer dependency" because, as far as I could tell, this was an implementation detail

This is not correct; a peer dependency is a well-documented concept in the npm ecosystem.

[DanKaplanSES]

• I removed the information about the canvas package being a "peer dependency" because, as far as I could tell, this was an implementation detail

This is not correct; a peer dependency is a well-documented concept in the npm ecosystem.

Hi @domenic Apologies for how I worded that; I didn't mean to imply peer dependencies are an obscure topic. Though, maybe I don't understand them as well as I thought I did. To be honest, I just learned about them today by reading this arti... Annnnd I just noticed you're the author. This is surreal. (It totally reminds me of a Futurama reference.)

Anyway, before I go, may I ask a clarifying question about the jsdom readme? It says:

To make this work, you need to include canvas as a dependency in your project, as a peer of jsdom.

But how does that bold part affect my behavior differently than if it was written like this:

To make this work, you need to include canvas as a dependency in your project.

Thanks for your time!