[mlir][docs] Add docs on canonicalizers being folders or patterns

mlir/docs/Canonicalization.md

@@ -275,3 +275,24 @@ Operation *MyDialect::materializeConstant(OpBuilder &builder, Attribute value,
   ...
 }
 ```
+
+### Choosing between implementing a Folder or a `RewritePattern`
+
+When implementing a new canonicalization, an important thing to think about is
+if the canonicalization should be a folder or a `RewritePattern`. The generally
+accepted policy is:
+
+*   If a transformation should be a canonicalizer is a different question from
+    if a transformation should be a folder or a `RewritePattern`. A
+    transformation is promoted to a canonicalization as defined by the General
+    Design of canonicalizations.
+
+*   If a transformation is a canonicalization, there are two ways to implement
+    it: As a Folder or as a `RewritePattern`, both of which are implementation
+    details of how a canonicalization exists, with the difference being folders
+    have restrictions on what they can do. A `RewritePattern` can implement any
+    transformation a folder can.
+
+*   A canonicalization should always be implemented as a Folder if it fits
+    the "local" definition of a folder, otherwise it should be implemented
+    as a `RewritePattern`.