Update the comment on Clang and MSVC's STL

Update the comment on Clang (on Windows) and MSVC's STL based on new
understanding of the issue.

Change-Id: I7a4d3b9b35956ac04d724b1e16ae283ab8517496
(cherry picked from commit 02ef05b22ad0d17ae49dbc054a69ca2ab60960fa)

build/cmake/aom_configure.cmake

diff --git a/build/cmake/aom_configure.cmake b/build/cmake/aom_configure.cmake
index 4a4a6ee..1ebfa9e 100644
--- a/build/cmake/aom_configure.cmake
+++ b/build/cmake/aom_configure.cmake
@@ -303,9 +303,9 @@
   if(CMAKE_CXX_COMPILER_ID STREQUAL "Clang"
      AND CMAKE_CXX_COMPILER_FRONTEND_VARIANT STREQUAL "GNU"
      AND CMAKE_CXX_SIMULATE_ID STREQUAL "MSVC")
-    # MSVC's STL requires C++14 as it's the compiler's default and minimum
-    # supported C++ version. However, clang (as opposed to clang-cl) in Visual
-    # Studio defaults to C++11.
+    # Microsoft's C++ Standard Library requires C++14 as it's MSVC's default and
+    # minimum supported C++ version. If Clang is using this Standard Library
+    # implementation, it cannot target C++11.
     require_cxx_flag_nomsvc("-std=c++14" YES)
   else()
     require_cxx_flag_nomsvc("-std=c++11" YES)