docs: add documentation for `LIBCXX_INCLUDE_TESTS`
docs: add documentation for `LIBCXX_INCLUDE_TESTS`

Add some missing documentation for the `LIBCXX_INCLUDE_TESTS` option.

Patch by Jean Heyd Meneide!

git-svn-id: 756ef344af921d95d562d9e9f9389127a89a6314@365154 91177308-0d34-0410-b5e6-96231b3b80d8

llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx,llvm-mirror/libcxx