[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:
+=====
+
+.. code-block:: text
+
+   clang-repl>%help
----------------
DavidSpickett wrote:

Actual clang-repl prints a space after the >, include that in all the examples. I know one of the existing ones doesn't, I think that one is just wrong.

Also easier to read with the space.

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