wit: migrate to only accepting `///` for doc comments

[sunfishcode]

Per the discussion in WebAssembly/component-model#286, there seems to be consensus for using /// as the documentation comment syntax.

This syntax is already recognized, so the plan for using /// is:

• Update the Wit printer to use it. Switch to /// for printing documentation comments. #1357
• After some time, add a deprecation warning for // comments pointing uses to ///.
• After some time, make the warning an error.