amd-pstate: Add missing documentation for `amd_pstate_prefcore_ranking`

`amd_pstate_prefcore_ranking` reflects the dynamic rankings of a CPU
core based on platform conditions.  Explicitly include it in the
documentation.

Reviewed-by: Gautham R. Shenoy <gautham.shenoy@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>