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

[Aaron Danen via cfe-commits]

aadanen wrote:

I thought about "Undoing Input" as the section header but I feel like %undo is pretty self explanatory. As far as "%help" and clang-repl --help go, I think those sound like nice ideas. Both would be useful for new people using clang-repl. 

As far as "%foobar" throwing an error like "invalid command, enter %help for list of commands" I think it would be okay. The only use of '%' I know of in cpp is the modulo operator which would never be the first character in a line

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