[libc++] Do not rely on use_system_cxx_lib to specify the path of the library to run against

This is already handled by setting cxx_runtime_root instead -- I don't
see a reason to have two ways of setting the runtime path of the library
we're running against.

Cr-Mirrored-From: https://chromium.googlesource.com/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: 21b0ec2fc6e17a0ba3921ed8146b7d410ce250c9
diff --git a/docs/TestingLibcxx.rst b/docs/TestingLibcxx.rst
index 78134bc..cc684e7 100644
--- a/docs/TestingLibcxx.rst
+++ b/docs/TestingLibcxx.rst
@@ -140,8 +140,7 @@
 .. option:: cxx_library_root=<path/to/lib/>
 
   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_cxx_lib is provided.
+  library folder of the build directory is used.
 
 
 .. option:: cxx_runtime_root=<path/to/lib/>
@@ -156,7 +155,10 @@
   **Default**: False
 
   Enable or disable testing against the installed version of libc++ library.
-  Note: This does not use the installed headers.
+  This impacts whether the ``with_system_cxx_lib`` Lit feature is defined or
+  not. The ``cxx_library_root`` and ``cxx_runtime_root`` parameters should
+  still be used to specify the path of the library to link to and run against,
+  respectively.
 
 .. option:: use_lit_shell=<bool>