[docs] Use doxygen to describe the field `StartAtCycle`. [NFCI]

[fhahn]

LGTM, thanks!

llvm/include/llvm/MC/MCSchedule.h

struct MCProcResourceDesc {
   }
 };
 
 /// Identify one of the processor resource kinds consumed by a particular
 /// scheduling class for the specified number of cycles.
 struct MCWriteProcResEntry {
   uint16_t ProcResourceIdx;
   uint16_t Cycles;
-  // Cycle at which the resource will be grabbed by an instruction,
-  // relatively to the cycle in which the instruction is issued
-  // (assuming no stalls inbetween).
+  /// Cycle at which the resource will be grabbed by an instruction,
+  /// relatively to the cycle in which the instruction is issued
+  /// (assuming no stalls inbetween).
   uint16_t StartAtCycle;
 
   bool operator==(const MCWriteProcResEntry &Other) const {
     return ProcResourceIdx == Other.ProcResourceIdx && Cycles == Other.Cycles &&
            StartAtCycle == Other.StartAtCycle;
   }
 };