Document CMAKE_CXX_STANDARD

This variable is a CMake community standard to set the C++
standard of a build. Document it in favor of the previous variable,
which stays as a legacy flag for existing projects.

https://cmake.org/cmake/help/v3.17/variable/CMAKE_CXX_STANDARD.html
diff --git a/docs/compiling.rst b/docs/compiling.rst
index c50c7d8..ea8d1ec 100644
--- a/docs/compiling.rst
+++ b/docs/compiling.rst
@@ -105,18 +105,14 @@
 available.  Note, however, that this default is subject to change: future
 pybind11 releases are expected to migrate to newer C++ standards as they become
 available.  To override this, the standard flag can be given explicitly in
-``PYBIND11_CPP_STANDARD``:
+`CMAKE_CXX_STANDARD <https://cmake.org/cmake/help/v3.17/variable/CMAKE_CXX_STANDARD.html>`_:
 
 .. code-block:: cmake
 
     # Use just one of these:
-    # GCC/clang:
-    set(PYBIND11_CPP_STANDARD -std=c++11)
-    set(PYBIND11_CPP_STANDARD -std=c++14)
-    set(PYBIND11_CPP_STANDARD -std=c++1z) # Experimental C++17 support
-    # MSVC:
-    set(PYBIND11_CPP_STANDARD /std:c++14)
-    set(PYBIND11_CPP_STANDARD /std:c++latest) # Enables some MSVC C++17 features
+    set(CMAKE_CXX_STANDARD 11)
+    set(CMAKE_CXX_STANDARD 14)
+    set(CMAKE_CXX_STANDARD 17) # Experimental C++17 support
 
     add_subdirectory(pybind11)  # or find_package(pybind11)