Document that TargetRegisterInfo::contains does not cover virtual registers.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@101903 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Target/TargetRegisterInfo.h

@@ -109,7 +109,7 @@ public:
   }
 
   /// contains - Return true if the specified register is included in this
-  /// register class.
+  /// register class.  This does not include virtual registers.
   bool contains(unsigned Reg) const {
     return RegSet.count(Reg);
   }