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

[Aaron Danen via cfe-commits]

aadanen wrote:

@DavidSpickett I've added %undo to the documentation and implemented %help. Would it be good to also add %help to the documentation?

To print stuff, I am just using printf(). Is this fine? It would be easy to switch to std::cout or it looks like there are some llvm print functions but I am not sure if its necessary/preferred to use them. 

As far as %something throwing a specific error about a bad %command, I feel like there are too few commands and they are too easy to type for it to be needed. Let me know if you think its needed and I will implement it.

Thanks
aadanen

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