ACPI: OSI: Update Documentation on custom _OSI strings

Originally, when there was some custom ASL needed for Linux, the
guidance for Linux-friendly OEMs was to introduced a new string in the
APCI tables and discuss it on the kernel mailing lists.

This lead to 3 strings being introduced.  Those strings now have the
original problems fixed in the kernel and have since been removed.
The kernel no longer advertises any custom Linux-* _OSI strings for
OEM use.

It was discovered that these removed strings were actually being abused
to detect Linux for unrelated reasons, so drop the encouragement for
new ones to be added.

Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>