[docs] Recommend assertions when testing.

Pointed out by Shoaib Meenai.

llvm-svn: 353008
diff --git a/llvm/docs/TestingGuide.rst b/llvm/docs/TestingGuide.rst
index 9908d57..949fecf 100644
--- a/llvm/docs/TestingGuide.rst
+++ b/llvm/docs/TestingGuide.rst
@@ -127,7 +127,7 @@
 
 .. code-block:: bash
 
-    % cmake -DCMAKE_BUILD_TYPE="Release"
+    % cmake -DCMAKE_BUILD_TYPE="Release" -DLLVM_ENABLE_ASSERTIONS=On
 
 If you have `Clang <http://clang.llvm.org/>`_ checked out and built, you
 can run the LLVM and Clang tests simultaneously using: