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

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.

Differential Revision: https://reviews.llvm.org/D149857

Author: jhuber6

libc/docs/gpu/testing.rst

@@ -5,6 +5,11 @@
 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: