docs: Document INTEL_FORCE_PROBE env-var

Signed-off-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: José Roberto de Souza <jose.souza@intel.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/29273>

docs/envvars.rst

@@ -660,6 +660,42 @@ Intel driver environment variables
    reduces time to collect metrics and hides infrequently used metrics.
    To enable all metrics, set value to 1.
 
+.. envvar:: INTEL_FORCE_PROBE
+
+   A comma-separated list of device probe override values. The basic
+   format is ``<pci-id>[,<pci-id>,...]``. The wildcard value of ``*``
+   will specify all known PCI IDs. If ``!`` precedes a PCI ID, or the
+   wildcard value, then the device support will be disabled. All
+   numbers are interpreted in base 16, and a ``0x`` prefix is
+   optional. Values specified later take precidence, so the wildcard
+   probably should only be used at the beginning.
+
+   Some examples :
+
+   ``1234,!abcd``
+      Device 0x1234 would be forced on and 0xabcd would be disabled.
+
+   ``1234,!*``
+      All devices are disabled since the wildcard appears later.
+
+   ``!*,0x1234``
+      All devices disabled except 0x1234 which is forced on.
+
+   ``*,!0x1234``
+      All devices are forced on, except 0x1234 which is disabled.
+
+   ``!0x1234,1234``
+      Support for device 0x1234 is forced on since the enable appears
+      later.
+
+   .. note::
+      If a device requires using :envvar:`INTEL_FORCE_PROBE` to force
+      it to load, then Mesa does not have full support for the device.
+      It may have limited, or possibly no functionality within Mesa at
+      this point. It is recommended to upgrade to a Mesa which does
+      not require :envvar:`INTEL_FORCE_PROBE` for the device as soon
+      as it is available.
+
 .. envvar:: INTEL_MEASURE
 
    Collects GPU timestamps over common intervals, and generates a CSV report