Update documentation

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Change-Id: I309880f5c7f77c58a8b186db320bbc0f0e634089

runtime/docs/api-reference/environment_variables.rst

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+.. meta::
+   :description: HSA runtime implementation
+   :keywords: ROCR, ROCm, library, tool, runtime
+
+.. _environment-variables:
+
+Environment variables
+========================
+
+The following table lists the most often used environment variables.
+
+.. list-table:: ROCR environment variables
+    :header-rows: 1
+    
+    * - Environment variable
+      - Possible values
+      - Description
+
+    * - HSA_ENABLE_SDMA
+      - 
+        * 0: Disabled
+        * 1: Enabled (default)
+      - This controls the use of DMA engines in all copy directions (Host-to-Device, Device-to-Host, Device-to-Device) when using the
+        ``hsa_memory_copy``, ``hsa_amd_memory_fill``, ``hsa_amd_memory_async_copy``, ``hsa_amd_memory_async_copy_on_engine`` APIs
+
+    * - HSA_ENABLE_PEER_SDMA
+      -
+        * 0: Disabled
+        * 1: Enabled (default)
+      - This controls the use of DMA engines for Device-to-Device copies when using the ``hsa_memory_copy``, ``hsa_amd_memory_async_copy``, ``hsa_amd_memory_async_copy_on_engine`` APIs
+
+.. note::
+    
+    The value of ``HSA_ENABLE_PEER_SDMA`` is ignored if ``HSA_ENABLE_SDMA`` is used to disable the use of DMA engines.