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

[David Spickett via cfe-commits]

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@@ -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:
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DavidSpickett wrote:

If you do want to build the docs then dig around in https://lab.llvm.org/buildbot/#/builders/45/builds/14873, you'll find the cmake options and ninja targets used.

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