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

[David Spickett via cfe-commits]

================
@@ -197,6 +197,20 @@ Comments:
    clang-repl> // Comments in Clang-Repl
    clang-repl> /* Comments in Clang-Repl */
 
+Help:
----------------
DavidSpickett wrote:

I think the doc needs to at some point introduce the concept of % commands, so how about adding a title like "Built in Commands" and in that, introducing the `%help` command? Something like:

```
clang-repl has some special commands that are of the form `%<something`. To list all these commands, use the `%help` command:
  clang-repl> %help
```
(no need to include the output, as it will change over time anyway.

The first mentioned % command is %lib I think, so this section could go before that.

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