[SYCL][Docs] Mark host as deprecated in SYCL_DEVICE_FILTER (#7117)

The host device has been removed and related APIs have been deprecated.
This includes the `host` device-type and backend in
`SYCL_DEVICE_FILTER`. This commit changes the documentation for
`SYCL_DEVICE_FILTER` to mention the deprecation of `host` in the filter
and remove any mention of the behavior of using the deprecated values
and interfaces.

Signed-off-by: Larsen, Steffen <[email protected]>

Author: steffenlarsen

sycl/doc/EnvironmentVariables.md

@@ -90,7 +90,7 @@ fixed order of properties in the pattern.
 This environment variable limits the SYCL RT to use only a subset of the system's devices. Setting this environment variable affects all of the device query functions (`platform::get_devices()` and `platform::get_platforms()`) and all of the device selectors. 
 
 The value of this environment variable is a comma separated list of filters, where each filter is a triple of the form "`backend`:`device_type`:`device_num`" (without the quotes). Each element of the triple is optional, but each filter must have at least one value. Possible values of `backend` are:
-- `host`
+- `host` (Deprecated)
 - `level_zero`
 - `opencl`
 - `cuda`
@@ -99,17 +99,17 @@ The value of this environment variable is a comma separated list of filters, whe
 - `*`
 
 Possible values of `device_type` are:
-- `host`
+- `host` (Deprecated)
 - `cpu`
 - `gpu`
 - `acc`
 - `*`
 
 `device_num` is an integer that indexes the enumeration of devices from the sycl-ls utility tool, where the first device in that enumeration has index zero in each backend. For example, `SYCL_DEVICE_FILTER=2` will return all devices with index '2' from all different backends. If multiple devices satisfy this device number (e.g., GPU and CPU devices can be assigned device number '2'), then default_selector will choose the device with the highest heuristic point.  When `SYCL_DEVICE_ALLOWLIST` is set, it is applied before enumerating devices and affects `device_num` values.
 
-Assuming a filter has all three elements of the triple, it selects only those devices that come from the given backend, have the specified device type, AND have the given device index. If more than one filter is specified, the RT is restricted to the union of devices selected by all filters. The RT does not include the `host` backend and the `host` device automatically unless one of the filters explicitly specifies the `host` device type. Therefore, `SYCL_DEVICE_FILTER=host` should be set to enforce SYCL to use the `host` device only. 
+Assuming a filter has all three elements of the triple, it selects only those devices that come from the given backend, have the specified device type, AND have the given device index. If more than one filter is specified, the RT is restricted to the union of devices selected by all filters.
 
-Note that all device selectors will throw an exception if the filtered list of devices does not include a device that satisfies the selector. For instance, `SYCL_DEVICE_FILTER=cpu,level_zero` will cause `host_selector()` to throw an exception. `SYCL_DEVICE_FILTER` also limits loading only specified plugins into the SYCL RT. In particular, `SYCL_DEVICE_FILTER=level_zero` will cause the `cpu_selector` to throw an exception since SYCL RT will only load the `level_zero` backend which does not support any CPU devices at this time. When multiple devices satisfy the filter (e..g, `SYCL_DEVICE_FILTER=gpu`), only one of them will be selected.
+Note that all device selectors will throw an exception if the filtered list of devices does not include a device that satisfies the selector. For instance, `SYCL_DEVICE_FILTER=cpu` will cause `gpu_selector()` to throw an exception. `SYCL_DEVICE_FILTER` also limits loading only specified plugins into the SYCL RT. In particular, `SYCL_DEVICE_FILTER=level_zero` will cause the `cpu_selector` to throw an exception since SYCL RT will only load the `level_zero` backend which does not support any CPU devices at this time. When multiple devices satisfy the filter (e..g, `SYCL_DEVICE_FILTER=gpu`), only one of them will be selected.
 
 ## `SYCL_REDUCTION_PREFERRED_WORKGROUP_SIZE`