Explicitly callout usage of bracket notation for item access in HashMap docs

[bpicolo]

Not a huge Rust user (Rust enthusiast!), but wasn't aware that square-bracket notation functioned for HashMaps. I see now that it's due to operator overloading via the Index interface, but newcomers to the language likely aren't going to be familiar with that.

Probably worth making it obvious that it's doable in the HashMap documentation? This example is currently the only place it shows up, somewhat sneakily

[varkor]

If added, it should also be explicitly noted that this notation does not work for updating HashMap elements.