Index: llvm/docs/CMake.rst
===================================================================
--- llvm/docs/CMake.rst
+++ llvm/docs/CMake.rst
@@ -431,6 +431,8 @@
   passed to invocations of both so that the project is built using libc++
   instead of stdlibc++. Defaults to OFF.
 
+  .. note:: LLVM_ENABLE_LIBCXX will not affect cmake checking CXX compiler, so it will fail when `libc++` is the only STL on system. In that case `-DCMAKE_CXX_FLAGS='-stdlib=libc++'` should be used.
+
 **LLVM_STATIC_LINK_CXX_STDLIB**:BOOL
   Statically link to the C++ standard library if possible. This uses the flag
   "-static-libstdc++", but a Clang host compiler will statically link to libc++