task: add more env var documentation

Author: jharlow-intel

docs/doc_sources/user_guides/environment_variables.rst

@@ -50,3 +50,61 @@ The value of the variable is a bit-mask, with the following supported values:
       - Enables tracing of PI calls
     * - ``-1``
       - Enables all levels of tracing
+
+Variable ``ZE_FLAT_DEVICE_HIERARCHY``
+--------------------------
+Allows users to define the device hierarchy model exposed by Level Zero driver implementation.
+
+.. list-table::
+    :header-rows: 1
+
+    * - Value
+      - Description
+    * - ``COMBINED``
+      - Enables all levels of tracing
+    * - ``COMPOSITE``
+      - Returns all devices that do not have a root-device.
+    * - ``FLAT``
+      - Returns all devices without sub-devices in a flat hierarchy. Enabled by default.
+
+Read more `here <https://oneapi-src.github.io/level-zero-spec/level-zero/latest/core/PROG.html#device-hierarchy>` and `here <https://www.intel.com/content/www/us/en/developer/articles/technical/flattening-gpu-tile-hierarchy.html>`.
+
+Variable ``ZE_AFFINITY_MASK``
+-------------------------------
+Allows users to mask specific devices from being used by SYCL applications.
+If we have ZE_FLAT_DEVICE_HIERARCHY set to COMPOSITE, we can have an AFFINITY of “1” for our application to only see device #1 - making system devices 0, and 2+, invisible. Etc.
+
+Read more `here <https://oneapi-src.github.io/level-zero-spec/level-zero/latest/core/PROG.html#affinity-mask>`.
+
+Variable ``ZE_ENABLE_PCI_ID_DEVICE_ORDER``
+-------------------------------
+Forces driver to report devices from lowest to highest PCI bus ID.
+
+.. list-table::
+    :header-rows: 1
+
+    * - Value
+      - Description
+    * - ``0``
+      - Disabled. Default value.
+    * - ``1``
+      - Enabled.
+
+Variable ``ZE_SHARED_FORCE_DEVICE_ALLOC``
+-------------------------------
+Forces all shared allocations into device memory
+
+.. list-table::
+    :header-rows: 1
+
+    * - Value
+      - Description
+    * - ``0``
+      - Disabled. Default value.
+    * - ``1``
+      - Enabled.
+
+-------------------------------
+Other relevant environment variables can be found here:
+- `Level Zero <https://intel.github.io/llvm/EnvironmentVariables.html>`
+- `OneAPI <https://oneapi-src.github.io/level-zero-spec/level-zero/latest/core/PROG.html#environment-variables>`