TableGen: Enumerate Schedule Model too.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@189839 91177308-0d34-0410-b5e6-96231b3b80d8

docs/WritingAnLLVMBackend.rst

@@ -952,6 +952,11 @@ XXXInstrInfo.h:
     int16_t getNamedOperandIdx(uint16_t Opcode, uint16_t NamedIndex);
   } // End namespace XXX
 
+Instruction itineraries can be queried using MCDesc::getSchedClass(). The
+value can be named by an enumemation in llvm::XXX::Sched namespace generated
+by TableGen in XXXGenInstrInfo.inc. The name of the schedule classes are
+the same as provided in XXXSchedule.td plus a default NoIntinerary class.
+
 Instruction Relation Mapping
 ----------------------------