Added information about array access type errors

[johnridesabike]

This adds a few paragraphs about how array access is handled by Belt. This seems to be a common source of confusion, and I haven’t seen it mentioned anywhere else in the docs.

I wasn’t sure where the best location for this would be, so I put it in the “implementation details” page.

[johnridesabike]

I added references to the how array access works in a few other parts of the docs. Let me know if you see anything else that should change!