swift234

[SourceKit] Convert Protocol docs to Markdown (#2392)

- Convert `Protocol.txt` to `Protocol.md`, and use valid Markdown
  syntax.
- Spruce up the documentation in the "code completion": more details on
  how to test via the REPL, clearer sentences, etc.
- Fix an error in which `sourcekitd-test -req=complete -offset=<offset>`
  used the incorrect option `-cc-offset`.