[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
+
+Undo:
+=====
+
----------------
DavidSpickett wrote:

I know it seems obvious what this does, but add a sentence at least, and a short example. Like:
```
The `%undo` command reverts the last input.

clang-repl> int a = 1; a
(int) 1
clang-repl> %undo
clang-repl> a
In file included from <<< inputs >>>:1:
input_line_2:1:1: error: use of undeclared identifier 'a'
    1 | a
      | ^
error: Parsing failed.
```
This one just shows that a can be printed before the undo, but not after.
  

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