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

[David Spickett via cfe-commits]

https://github.com/DavidSpickett commented:

So this is great, just some small things to fix.

Next you need to add tests for:
* %help
* %lib with no argument
* %rubbish error message

I would do all those as their own test files. You could add the %lib bit to one of the existing tests, but each file seems to have a specific thing it's checking, so new files seems ok.

The maintainer can disagree later if not.

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