[GPU][NFC] Update comments for MMAAttr attribute (#18339)

The revision redirects the layout comment to the enum file which reduces
the maintenance burden. People wont miss to update comments like this.
IMO, it is also the naming convention, so we don't need to spell it
loud.

The revision also make the "section comment" consistent.

compiler/src/iree/compiler/Codegen/Dialect/GPU/IR/IREEGPUAttrs.td

@@ -172,6 +172,7 @@ class IREEGPU_MmaVectorLayoutAttr<string attrname, string mmaintrinsic> :
 
 //===----------------------------------------------------------------------===//
 // MMA intrinsic
+//===----------------------------------------------------------------------===//
 
 class IREEGPU_MmaEnumAttr<EnumAttrInfo enumInfo, string name = "">
   : EnumAttr<IREEGPU_Dialect, enumInfo, name>;
@@ -186,12 +187,8 @@ def IREEGPU_MMAAttr : IREEGPU_MmaVectorLayoutAttr<"MMA", "MMAIntrinsicAttr"> {
   let description = !strconcat(baseDescription, [{
     This mma variant describes configurations for MMA ops. The |intrinsic|
     field specifies which particular MMA intrinsic this refers to, with each
-    intrinsic implicating a specific MNK shape and operand types. The intrinsic
-    enum name describes these fields as
-
-    <InputType>_MxNxK_<CType>
-
-    Where the element type for the `A` and `B` matrices are both `InputType`.
+    intrinsic implicating a specific MNK shape and operand types.
+    See IREEGPUEnums.td for the definition of the intrinsics.
   }]);
 
   let hasCustomAssemblyFormat = 1;
@@ -230,6 +227,7 @@ def IREEGPU_MMAOpsArrayAttr : ArrayOfAttr<
 
 //===----------------------------------------------------------------------===//
 // MMA schedule
+//===----------------------------------------------------------------------===//
 
 def IREEGPU_MmaScheduleAttr : AttrDef<IREEGPU_Dialect, "MMASchedule"> {
   let mnemonic = "mma_schedule";
@@ -259,6 +257,7 @@ def IREEGPU_MmaScheduleAttr : AttrDef<IREEGPU_Dialect, "MMASchedule"> {
 
 //===----------------------------------------------------------------------===//
 // Workgroup processor level description
+//===----------------------------------------------------------------------===//
 
 def IREEGPU_TargetWgpAttr : AttrDef<IREEGPU_Dialect, "TargetWgp"> {
   let summary = "Workgroup processor level target description";