[RISCV] Simplify the description for ssaia and smaia. (#77870)

It feels more important to expand out Advanced Interrupt Architecture
for users than to have a description that explains how one extension is
different from the other.

Author: topperc

llvm/lib/Target/RISCV/RISCVFeatures.td

@@ -711,16 +711,13 @@ def NoHasStdExtZicfiss : Predicate<"!Subtarget->hasStdExtZicfiss()">;
 
 def FeatureStdExtSmaia
     : SubtargetFeature<"smaia", "HasStdExtSmaia", "true",
-                       "'Smaia' (Smaia encompasses all added CSRs and all "
-                       "modifications to interrupt response behavior that the "
-                       "AIA specifies for a hart, over all privilege levels.)",
-                       []>;
+                       "'Smaia' (Advanced Interrupt Architecture Machine "
+                       "Level)", []>;
 
 def FeatureStdExtSsaia
     : SubtargetFeature<"ssaia", "HasStdExtSsaia", "true",
-                       "'Ssaia' (Ssaia is essentially the same as Smaia except "
-                       "excluding the machine-level CSRs and behavior not "
-                       "directly visible to supervisor level.)", []>;
+                       "'Ssaia' (Advanced Interrupt Architecture Supervisor "
+                       "Level)", []>;
 
 def HasHalfFPLoadStoreMove
     : Predicate<"Subtarget->hasHalfFPLoadStoreMove()">,