docs/kernel-parameters: Update descriptions for "mitigations=" param with retbleed

Updates descriptions for "mitigations=off" and "mitigations=auto,nosmt"
with the respective retbleed= settings.

Signed-off-by: Eiichi Tsukata <[e-mail]>
Signed-off-by: Borislav Petkov <[e-mail]>
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]

Documentation/admin-guide/kernel-parameters.txt

···
 					       no_entry_flush [PPC]
 					       no_uaccess_flush [PPC]
 					       mmio_stale_data=off [X86]
+					       retbleed=off [X86]
 
 				Exceptions:
 					       This does not have any effect on
···
 					       mds=full,nosmt [X86]
 					       tsx_async_abort=full,nosmt [X86]
 					       mmio_stale_data=full,nosmt [X86]
+					       retbleed=auto,nosmt [X86]
 
 	mminit_loglevel=
 			[KNL] When CONFIG_DEBUG_MEMORY_INIT is set, this