Add cxx_runtime_root options for testing against a different libc++ at runtime

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@266855 91177308-0d34-0410-b5e6-96231b3b80d8

docs/TestingLibcxx.rst

diff --git a/docs/TestingLibcxx.rst b/docs/TestingLibcxx.rst
index 98162d9..5aecc07 100644
--- a/docs/TestingLibcxx.rst
+++ b/docs/TestingLibcxx.rst
@@ -118,11 +118,20 @@
   Specify the libc++ headers that are tested. By default the headers in the
   source tree are used.
 
-.. option:: libcxx_library=<path/to/libc++.so>
+.. option:: cxx_library_root=<path/to/lib/>
 
-  Specify the libc++ library that is tested. By default the library in the
-  build directory is used. This option cannot be used when use_system_lib is
-  provided.
+  Specify the directory of the libc++ library to be tested. By default the
+  library folder of the build directory is used. This option cannot be used
+  when use_system_lib is provided.
+
+
+.. option:: cxx_runtime_root=<path/to/lib/>
+
+  Specify the directory of the libc++ library to use at runtime. This directory
+  is not added to the linkers search path. This can be used to compile tests
+  against one version of libc++ and run them using another. The default value
+  for this option is `cxx_library_root`. This option cannot be used
+  when use_system_lib is provided.
 
 .. option:: use_system_lib=<bool>