amdgpu/pm: Clarify documentation of error handling in send_smc_mesg

Clarify the smu_cmn_send_smc_msg_with_param documentation to mention two
cases exist where messages are silently dropped with no error returned.
These cases occur in unusual situations where either:
 1. the message type is not allowed to a virtual GPU, or
 2. a PCI recovery is underway and the HW is not yet in sync with the SW

For more details see
 commit 4ea5081c ("drm/amd/powerplay: enable SMC message filter")
 commit bf36b52e ("drm/amdgpu: Avoid accessing HW when suspending SW state")

(v2)
  Reworked with suggestions from Luben & Paul

(v3)
  Updated wording as per Luben's feedback
  Corrected error stating all messages denied on virtual GPU
  (each GPU has mask of which messages are allowed)

Signed-off-by: Darren Powell <darren.powell@amd.com>
Reviewed-by: Luben Tuikov <luben.tuikov@amd.com>