Fix/Clarify parts of MLIR toy tutorial chapter 6+7

[matthiaskramm]

Summary

• add missing comma.
• remove "having to register them here" phrasing, since register it is what we're doing, which made the comment a bit confusing.
• remove duplicate code.
• clarify link to chapter 3, since "folder" doesn't appear in that chapter.

[rriddle]

mlir/docs/Tutorials/Toy/Ch-7.md

404

nit: Using %0 from above.

[matthiaskramm]

Thanks for the review, River! Fixed the nit.

mlir/docs/Tutorials/Toy/Ch-6.md

 by relying on [transitive lowering](../../../getting_started/Glossary.md#transitive-lowering).
 
 ```c++
   mlir::OwningRewritePatternList patterns;
   mlir::populateAffineToStdConversionPatterns(patterns, &getContext());
   mlir::populateLoopToStdConversionPatterns(patterns, &getContext());
   mlir::populateStdToLLVMConversionPatterns(typeConverter, patterns);
 
-  // The only remaining operation to lower from the `toy` dialect, is the
+  // The only remaining operation, to lower from the `toy` dialect, is the
   // PrintOp.
   patterns.insert<PrintOpLowering>(&getContext());
 ```
 
 ### Full Lowering
 
 We want to completely lower to LLVM, so we use a `FullConversion`. This ensures
 that only legal operations will remain after the conversion.

mlir/docs/Tutorials/Toy/Ch-7.md

 symbol registry file
 [DialectSymbolRegistry](https://github.com/llvm/llvm-project/blob/master/mlir/include/mlir/IR/DialectSymbolRegistry.def).
 
 ```c++
 DEFINE_SYM_KIND_RANGE(LINALG) // Linear Algebra Dialect
 DEFINE_SYM_KIND_RANGE(TOY)    // Toy language (tutorial) Dialect
 
 // The following ranges are reserved for experimenting with MLIR dialects in a
-// private context without having to register them here.
+// private context.
 DEFINE_SYM_KIND_RANGE(PRIVATE_EXPERIMENTAL_0)
 ```
 
 These definitions will provide a range in the Type::Kind enum to use when
 defining the derived types.
 
 ```c++
 /// Create a local enumeration with all of the types that are defined by Toy.
```mlir
   ] : !toy.struct<tensor<*xf64>>
 ```
 
 ##### `toy.struct_access`
 
 This new operation materializes the Nth element of a `struct` value.
 
 ```mlir
-  %0 = toy.struct_constant [
-    dense<[[1.0, 2.0, 3.0], [4.0, 5.0, 6.0]]> : tensor<2x3xf64>
-  ] : !toy.struct<tensor<*xf64>>
+  // %0 from above
   %1 = toy.struct_access %0[0] : !toy.struct<tensor<*xf64>> -> tensor<*xf64>
 ```
 
 With these operations, we can revisit our original example:
 
 ```toy
 struct Struct {
   var a;
func @main() {
     %5 = toy.mul %2, %4 : tensor<*xf64>
     toy.print %5 : tensor<*xf64>
     toy.return
   }
 }
 ```
 
 We have several `toy.struct_access` operations that access into a
-`toy.struct_constant`. As detailed in [chapter 3](Ch-3.md), we can add folders
-for these `toy` operations by setting the `hasFolder` bit on the operation
-definition and providing a definition of the `*Op::fold` method.
+`toy.struct_constant`. As detailed in [chapter 3](Ch-3.md) (FoldConstantReshape),
+we can add folders for these `toy` operations by setting the `hasFolder` bit
+on the operation definition and providing a definition of the `*Op::fold`
+method.
 
 ```c++
 /// Fold constants.
 OpFoldResult ConstantOp::fold(ArrayRef<Attribute> operands) { return value(); }
 
 /// Fold struct constants.
 OpFoldResult StructConstantOp::fold(ArrayRef<Attribute> operands) {
   return value();