[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:
+
+Help:
+=====
----------------
DavidSpickett wrote:

Figure out how to make these titles one level lower than the "built in commands" bit. In markdown this is done like:
```
# this is a heading
## this is a sub heading
```
In RST it's what character is used for the underline. Here's the best reference I could find - https://bashtage.github.io/sphinx-material/rst-cheatsheet/rst-cheatsheet.html#section-structure

So I think since this has used ==== for the main title, any other character would create a sub heading.

Can you tell that I prefer markdown? :) RST is very powerful but overkill for most of the docs we have.

GitHub can preview RST pretty well, but not perfectly. It should be enough for this title rendering though.


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