Tidy up some docs.

[lattner]

Summary

Depends on D99127.

[lattner]

Ooops, this was intended to be part of the previous patch. Sorry about the noise.

mlir/docs/Tutorials/Toy/Ch-5.md

 Now we can prepare the list of patterns to use during the lowering process:
 
 ```c++
 void ToyToAffineLoweringPass::runOnFunction() {
   ...
 
   // Now that the conversion target has been defined, we just need to provide
   // the set of patterns that will lower the Toy operations.
-  mlir::RewritePatternSet patterns;
+  mlir::RewritePatternSet patterns(&getContext());
   patterns.add<..., TransposeOpLowering>(&getContext());
 
   ...
 ```
 
 ## Partial Lowering
 
 Once the patterns have been defined, we can perform the actual lowering. The

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

 Now that the conversion target has been defined, we need to provide the patterns
 used for lowering. At this point in the compilation process, we have a
 combination of `toy`, `affine`, and `std` operations. Luckily, the `std` and
 `affine` dialects already provide the set of patterns needed to transform them
 into LLVM dialect. These patterns allow for lowering the IR in multiple stages
 by relying on [transitive lowering](../../../getting_started/Glossary.md#transitive-lowering).
 
 ```c++
-  mlir::RewritePatternSet patterns;
+  mlir::RewritePatternSet patterns(&getContext());
   mlir::populateAffineToStdConversionPatterns(patterns, &getContext());
   mlir::populateLoopToStdConversionPatterns(patterns, &getContext());
   mlir::populateStdToLLVMConversionPatterns(typeConverter, patterns);
 
   // The only remaining operation, to lower from the `toy` dialect, is the
   // PrintOp.
   patterns.add<PrintOpLowering>(&getContext());
 ```