[mlir][docs] Add docs on canonicalizers being folders or patterns (llvm#129517)

If a transformation should be a canonicalization is an orthogonal
question to if a transformation should be implemented as a
`RewritePattern` or a `fold` method. The later is an implementation
detail.

This patch adds a suggestion to always implement a canonicalization as a
`fold` pattern if possible, as they are a restricted subset of a
`RewritePattern`.

This has been a common source of confusion, as to when to implement a
canonicalization as a fold method or a RewritePattern.

Author: Groverkss

mlir/docs/Canonicalization.md

@@ -275,3 +275,8 @@ Operation *MyDialect::materializeConstant(OpBuilder &builder, Attribute value,
   ...
 }
 ```
+
+### When to use the `fold` method vs `RewriterPattern`s for canonicalizations
+
+A canonicalization should always be implemented as a `fold` method if it can
+be, otherwise it should be implemented as a `RewritePattern`.