[libc][Docs] Add warning about running GPU tests with parallelism

[jhuber6]

Summary

Now that a large amount of GPU tests can be run in parallel, we should
document spurious failures. It is a well-known issue that launching many
GPU applications in parallel can lead to various problems, the worst of
which being an indefinite hang.

libc/docs/gpu/testing.rst

 .. _libc_gpu_testing:
 
 
 ============================
 Testing the GPU libc library
 ============================
 
+.. note::
+   Running GPU tests with high parallelism is likely to cause spurious failures,
+   out of resource errors, or indefinate hangs. limiting the number of threads 
+   used while testing is highly recommended.
+
 .. contents:: Table of Contents
   :depth: 4
   :local:
 
 Testing Infrastructure
 ======================
 
 The testing support in LLVM's libc implementation for GPUs is designed to mimic