docs: add documentation for `LIBCXX_INCLUDE_TESTS`

Add some missing documentation for the `LIBCXX_INCLUDE_TESTS` option.

Patch by Jean Heyd Meneide!

llvm-svn: 365154
Cr-Mirrored-From: sso://chromium.googlesource.com/_direct/external/github.com/llvm/llvm-project
Cr-Mirrored-Commit: bce9d8b138ca823d4ed7eacd0642980ca5172f74
diff --git a/docs/BuildingLibcxx.rst b/docs/BuildingLibcxx.rst
index 06f8960..2ec546d 100644
--- a/docs/BuildingLibcxx.rst
+++ b/docs/BuildingLibcxx.rst
@@ -288,6 +288,12 @@
 
   Build libc++ with run time type information.
 
+.. option:: LIBCXX_INCLUDE_TESTS:BOOL
+
+  **Default**: ``ON`` (or value of ``LLVM_INCLUDE_DIR``)
+
+  Build the libc++ tests.
+
 .. option:: LIBCXX_INCLUDE_BENCHMARKS:BOOL
 
   **Default**: ``ON``