Documentation/kernel-parameters: Add spec_rstack_overflow to mitigations=off

mitigations=off disables the SRSO mitigation too. Add it to the list.
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20240118163600.17857-1-bp@alien8.de

Documentation/admin-guide/kernel-parameters.txt

@@ -3399,6 +3399,7 @@
 					       nospectre_v1 [X86,PPC]
 					       nospectre_v2 [X86,PPC,S390,ARM64]
 					       retbleed=off [X86]
+					       spec_rstack_overflow=off [X86]
 					       spec_store_bypass_disable=off [X86,PPC]
 					       spectre_v2_user=off [X86]
 					       srbds=off [X86,INTEL]