Add doc comment for updateLRRestored

Author: ostannard

llvm/lib/Target/ARM/ARMFrameLowering.h

@@ -59,7 +59,10 @@ class ARMFrameLowering : public TargetFrameLowering {
   void determineCalleeSaves(MachineFunction &MF, BitVector &SavedRegs,
                             RegScavenger *RS) const override;
 
+  /// Update the IsRestored flag on LR if it is spilled, based on the return
+  /// instructions.
   void updateLRRestored(MachineFunction &MF) const;
+
   void processFunctionBeforeFrameFinalized(
       MachineFunction &MF, RegScavenger *RS = nullptr) const override;