When a user hovers over a method name in a function call, we should pop up a bubble showing the documentation for that method, if it exists.

Compare:
External (no bubble): https://cs.chromium.org/chromium/src/base/command_line.cc?l=89&rcl=a843b673246d7978981ce847f5a6250790ceec39
Internal (bubble): http://shortn/_wcA3B5XwnD

I'm not sure where the info for those bubbles comes from. It looks like the hover first triggers a request to kythe to get the location of the function definition, and then a second request to a different service. So it might mean running+maintaining+populating a wholly separate pipeline. But it's worth looking into.