docs/envvars: document some vulkan loader env vars

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28516>

docs/envvars.rst

@@ -1842,3 +1842,40 @@ Freedreno driver environment variables
 
 Other Gallium drivers have their own environment variables. These may
 change frequently so the source code should be consulted for details.
+
+
+Vulkan loader environment variables
+-----------------------------------
+
+These variable are handled by `Khronos' Vulkan loader
+<https://github.com/KhronosGroup/Vulkan-Loader>`__, *not by Mesa*, but they
+are documented here as we reference them in other places in our docs.
+
+.. envvar:: VK_DRIVER_FILES
+
+   Force the loader to use the specific driver JSON files. The value contains
+   a list of delimited full path listings to driver JSON Manifest files
+   and/or paths to folders containing driver JSON files.
+
+   See `Vulkan loader docs on environment variables`_.
+
+.. envvar:: VK_LOADER_LAYERS_ENABLE
+
+    A comma-delimited list of globs to search for in known layers and used to
+    select only the layers whose layer name matches one or more of the
+    provided globs.
+    Known layers are those which are found by the loader taking into account
+    default search paths and other environment variables (like VK_LAYER_PATH).
+
+   See `Vulkan loader docs on environment variables`_.
+
+.. envvar:: VK_ICD_FILENAMES
+
+   `Deprecated`_, replaced by :envvar:`VK_DRIVER_FILES`.
+
+.. envvar:: VK_INSTANCE_LAYERS
+
+   `Deprecated`_, replaced by :envvar:`VK_LOADER_LAYERS_ENABLE`.
+
+.. _Vulkan loader docs on environment variables: https://github.com/KhronosGroup/Vulkan-Loader/blob/main/docs/LoaderInterfaceArchitecture.md#table-of-debug-environment-variables
+.. _Deprecated: https://github.com/KhronosGroup/Vulkan-Loader/blob/main/docs/LoaderInterfaceArchitecture.md#deprecated-environment-variables