Documentation: Add documentation about class interface for platform profiles

The class interface allows changing multiple platform profiles on a system
to different values. The semantics of it are similar to the legacy
interface.

Reviewed-by: Armin Wolf <[e-mail]>
Tested-by: Mark Pearson <[e-mail]>
Reviewed-by: Mark Pearson <[e-mail]>
Reviewed-by: Ilpo Järvinen <[e-mail]>
Signed-off-by: Mario Limonciello <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Ilpo Järvinen <[e-mail]>

Documentation/ABI/testing/sysfs-platform_profile

···
 		source such as e.g. a hotkey triggered profile change handled
 		either directly by the embedded-controller or fully handled
 		inside the kernel.
+
+		This file may also emit the string 'custom' to indicate
+		that multiple platform profiles drivers are in use but
+		have different values.  This string can not be written to
+		this interface and is solely for informational purposes.

Documentation/userspace-api/sysfs-platform_profile.rst

···
  1. Explain why the existing profile names cannot be used.
  2. Add the new profile name, along with a clear description of the
     expected behaviour, to the sysfs-platform_profile ABI documentation.
+
+"Custom" profile support
+========================
+The platform_profile class also supports profiles advertising a "custom"
+profile. This is intended to be set by drivers when the setttings in the
+driver have been modified in a way that a standard profile doesn't represent
+the current state.
+
+Multiple driver support
+=======================
+When multiple drivers on a system advertise a platform profile handler, the
+platform profile handler core will only advertise the profiles that are
+common between all drivers to the ``/sys/firmware/acpi`` interfaces.
+
+This is to ensure there is no ambiguity on what the profile names mean when
+all handlers don't support a profile.
+
+Individual drivers will register a 'platform_profile' class device that has
+similar semantics as the ``/sys/firmware/acpi/platform_profile`` interface.
+
+To discover which driver is associated with a platform profile handler the
+user can read the ``name`` attribute of the class device.
+
+To discover available profiles from the class interface the user can read the
+``choices`` attribute.
+
+If a user wants to select a profile for a specific driver, they can do so
+by writing to the ``profile`` attribute of the driver's class device.
+
+This will allow users to set different profiles for different drivers on the
+same system. If the selected profile by individual drivers differs the
+platform profile handler core will display the profile 'custom' to indicate
+that the profiles are not the same.
+
+While the ``platform_profile`` attribute has the value ``custom``, writing a
+common profile from ``platform_profile_choices`` to the platform_profile
+attribute of the platform profile handler core will set the profile for all
+drivers.