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

[Aaron Danen via cfe-commits]

aadanen wrote:

1. printing stuff on clang-repl start causes several tests to fail
2. need to not parse %lib as a bad % command. Maybe should add %lib as list of % commands?

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