[libc][NFC][Docs] Update some GPU testing information

Summary:
This comment is outddated and can be removed, also mention an option for
limiting parallelism during tests in the documentation.

Author: jhuber6

libc/CMakeLists.txt

@@ -288,7 +288,6 @@ endif()
 # The lib and test directories are added at the very end as tests
 # and libraries potentially draw from the components present in all
 # of the other directories.
-# TODO: Add testing support for the libc GPU target.
 add_subdirectory(lib)
 if(LLVM_INCLUDE_TESTS)
   add_subdirectory(test)

libc/docs/gpu/testing.rst

@@ -8,7 +8,7 @@ Testing the GPU libc library
 .. note::
    Running GPU tests with high parallelism is likely to cause spurious failures,
    out of resource errors, or indefinite hangs. limiting the number of threads
-   used while testing is highly recommended.
+   used while testing using ``LIBC_GPU_TEST_JOBS=<N>`` is highly recommended.
 
 .. contents:: Table of Contents
   :depth: 4