[clang] [clang-repl] adding documentation and information for %commands (PR #150348)

[David Spickett via cfe-commits]

DavidSpickett wrote:

Please update the PR description to reflect the current changes.

@vgvassilev what do you think of this?

Only question I have is whether there's a special way to print non-error output, or whether stdout is fine? In theory `%help` is just like `printf("..."); but it prints some set text.

https://github.com/llvm/llvm-project/pull/150348