Docs: update unified memory system requirements

docs/how-to/hip_runtime_api/memory_management/unified_memory.rst

@@ -74,10 +74,13 @@ System requirements
 ================================================================================
 
 Unified memory is supported on Linux by all modern AMD GPUs from the Vega
-series onward. Unified memory management can be achieved by explicitly
-allocating managed memory using :cpp:func:`hipMallocManaged` or marking variables
-with the ``__managed__`` attribute, or, for the latest GPUs, with the normal
-system allocator, as shown in the following table.
+series onward, as shown in the following table. Unified memory management can
+be achieved by explicitly allocating managed memory using
+:cpp:func:`hipMallocManaged` or marking variables with the ``__managed__``
+attribute. For the latest GPUs, with a Linux kernel that supports
+`Heterogeneous Memory Management (HMM)
+<https://www.kernel.org/doc/html/latest/mm/hmm.html>`_, the normal system
+allocator can be used.
 
 .. list-table:: Supported Unified Memory Allocators by GPU architecture
     :widths: 40, 25, 25
@@ -107,9 +110,9 @@ system allocator, as shown in the following table.
 
 ❌: **Unsupported**
 
-:sup:`1` Works only with ``XNACK=1``. First GPU access causes recoverable
-page-fault. For more details, visit
-`GPU memory <https://rocm.docs.amd.com/en/latest/conceptual/gpu-memory.html#xnack>`_.
+:sup:`1` Works only with ``XNACK=1`` and kernels with HMM support. First GPU
+access causes recoverable page-fault. For more details, visit `GPU memory
+<https://rocm.docs.amd.com/en/latest/conceptual/gpu-memory.html#xnack>`_.
 
 .. _unified memory allocators:
 
@@ -131,14 +134,14 @@ system requirements` and :ref:`checking unified memory management support`.
 
 - **System allocated unified memory**
 
-  Starting with CDNA2, the ``new`` and `malloc()`` system allocators allow
+  Starting with CDNA2, the ``new`` and ``malloc()`` system allocators allow
   you to reserve unified memory. The system allocator is more versatile and
   offers an easy transition for code written for CPUs to HIP code as the
   same system allocation API is used.
 
 To ensure the proper functioning of system allocated unified memory on supported
 GPUs, it is essential to configure the environment variable ``XNACK=1`` and use
-a kernel that supports `Heterogeneous Memory Management (HMM)
+a kernel that supports `HMM
 <https://www.kernel.org/doc/html/latest/mm/hmm.html>`_. Without this
 configuration, the behavior will be similar to that of systems without HMM
 support. For more details, visit
@@ -383,6 +386,7 @@ explicit memory management example is presented in the last tab.
                 *c = *a + *b;
             }
 
+            // This example requires HMM support and the environment variable HSA_XNACK needs to be set to 1
             int main() {
                 // Allocate memory for a, b, and c.
                 int *a = new int[1];