[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`.
