Standardize getter docblocks in node module

As described in #2471485: [Meta] Standardize getter docblocks we want to standardize on using "Gets" instead of "Returns" on getter docblocks, matching the outside world and making core internally consistent. In general, we make the verb match the one in the method name.

The node module is used by many to see how to do stuff, so it makes sense that we want this to be consistent.