[OpenCL][Docs] Adding builtins requires adding to both now

As we are trying to reach parity between opencl-c.h and
-fdeclare-opencl-builtins, ensure the documentation mentions that new
builtins should be added to both.

Reviewed by: Anastasia Stulova

clang/docs/OpenCLSupport.rst

@@ -252,7 +252,7 @@ with :ref:`-cl-ext <opencl_cl_ext>` command-line flags.
 **Library functionality**
 
 If an extension adds functionality that does not modify standard language
-parsing it should not require modifying anything other than header files or
+parsing it should not require modifying anything other than header files and
 ``OpenCLBuiltins.td`` detailed in :ref:`OpenCL builtins <opencl_builtins>`.
 Most commonly such extensions add functionality via libraries (by adding
 non-native types or functions) parsed regularly. Similar to other languages this