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

[David Spickett via cfe-commits]

================
@@ -189,21 +202,40 @@ Using Dynamic Library:
    clang++-17  -c -o print.o print.cpp
    clang-17 -shared print.o -o print.so
 
-Comments:
-=========
+Undo:
+=====
+The `%undo` command reverts the previous input.
----------------
DavidSpickett wrote:

I would say "undoes". I know it seems redundant but it means you aren't leaving any mystery about what is a revert versus an undo.

The purpose of these docs is visibility more than help tbh, so it's fine if the description is formulaic.

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