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

[David Spickett via cfe-commits]

DavidSpickett wrote:

Sorry, I didn't realise this was a PR! So tunnel visioned on the comment notification. So if any of what I said made not much sense, that's why :)

Will review this today.

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