update doc for amrex::Abort on GPU (AMReX-Codes#3605)

The docs on the behavior of amrex::Abort and related functions appear to
be out of date, so this clarifies that when running on GPU these
functions do not do anything when called from within GPU kernels unless
you compile in debug mode or with `USE_ASSERTION=TRUE`.

Docs/sphinx_documentation/source/Basics.rst

@@ -2812,3 +2812,6 @@ Backtrace files are produced by AMReX signal handler by default when
 segfault occurs or ``Abort`` is called.  If the application does not
 want AMReX to handle this, ``ParmParse`` parameter
 `amrex.signal_handling=0` can be used to disable it.
+
+See :ref:`sec:gpu:assertion` for considerations on using these functions in
+GPU-enabled code.

Docs/sphinx_documentation/source/GPU.rst

@@ -1553,9 +1553,13 @@ Assertions and Error Checking
 To help debugging, we often use :cpp:`amrex::Assert` and
 :cpp:`amrex::Abort`.  These functions are GPU safe and can be used in
 GPU kernels.  However, implementing these functions requires additional
-GPU registers, which will reduce overall performance.  Therefore, it
-is preferred to implement such calls in debug mode only by wrapping the
-calls using ``#ifdef AMREX_DEBUG``.
+GPU registers, which will reduce overall performance.  Therefore, by
+default these functions and the macro ``AMREX_ALWAYS_ASSERT`` are no-ops
+for optimized builds (e.g., ``DEBUG=FALSE`` using the GNU Make build
+system) when called from kernels run on GPU. Calls to these functions from
+GPU kernels are active for debug builds and can optionally be activated
+at compile time for optimized builds (e.g., ``DEBUG=FALSE`` and
+``USE_ASSERTION=TRUE`` using the GNU Make build system).
 
 In CPU code, :cpp:`AMREX_GPU_ERROR_CHECK()` can be called
 to check the health of previous GPU launches.  This call