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

[David Spickett via cfe-commits]

================
@@ -159,8 +159,21 @@ Lamdas:
    clang-repl> welcome();
    Welcome to REPL
 
-Using Dynamic Library:
-======================
+Built in Commands:
+==================
+clang-repl has some special commands that are of the form `%<something>`. To list all these commands, use the `%help` command:
----------------
DavidSpickett wrote:

In RST, plain text markers use double backticks as opposed to markdown's single.

I can tell because A: I've struggled with RST enough to grow to loathe it, and B: if in the github review interface I go to the top right "..." then "view file", GitHub will render it and you can check from there.

The other way is to build the docs locally, which isn't that hard, but GitHub's preview should be ok for this stuff.

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