[MLIR] Update DRR doc with returnType directive

[Mogball]

Summary

Add missing documentation.

[Chia-hungDuan]

Thanks!

[jpienaar]

Thanks

mlir/docs/DeclarativeRewrites.md

 where all `$symbol` should be bound previously in the pattern and one optional
 string may be specified as an attribute. The following locations are created:
 
 *   If only 1 symbol is specified then that symbol's location is used,
 *   If multiple are specified then a fused location is created;
 *   If no symbol is specified then string must be specified and a NamedLoc is
     created instead;
 
-`location` must be used as the last argument to an op creation. For example,
+`location` must be used as a trailing argument to an op creation. For example,
 
 ```tablegen
 def : Pat<(LocSrc1Op:$src1 (LocSrc2Op:$src2 ...),
           (LocDst1Op (LocDst2Op ..., (location $src2)), (location "outer"))>;
 ```
 
 In the above pattern, the generated `LocDst2Op` will use the matched location of
 `LocSrc2Op` while the root `LocDst1Op` node will used the named location

 
 ```tablegen
 def : Pat<(Foo $input), (replaceWithValue $input)>;
 ```
 
 The above pattern removes the `Foo` and replaces all uses of `Foo` with
 `$input`.
 
+### `returnType`
+
+The `returnType` directive allows patterns to directly specify return types for
+replacement ops that lack return type inference with op traits or user-defined
+builders with return type deduction.
+
+The `returnType` directive must be used as a trailing argument to a node
+describing a replacement op. The directive comes in three forms:
+
+* `(returnType $value)`: copy the type of the operand or result bound to
+  `value`.
+* `(returnType "$_builder.getI32Type()")`: a string literal embedding C++. The
+  embedded snippet is expected to return a `Type` or a `TypeRange`.
+* `(returnType (NativeCodeCall<"myFunc($0)"> $value))`: a DAG node with a native
+  code call that can be passed any bound variables arguments.
+
+Specify multiple return types with a mix of any of the above. Example:
+
+```tablegen
+def : Pat<(SourceOp $arg0, $arg1),
+          (OpA $arg0, (TwoResultOp:$res__1 $arg1,
+                         (returnType $arg1, "$_builder.getI64Type()")))>;
+```
+
+Explicitly-specified return types will take precedence over return types
+inferred from op traits or user-defined builders. The return types of values
+replacing root op results cannot be overridden.
+
 ## Debugging Tips
 
 ### Run `mlir-tblgen` to see the generated content
 
 TableGen syntax sometimes can be obscure; reading the generated content can be a
 very helpful way to understand and debug issues. To build `mlir-tblgen`, run
 `cmake --build . --target mlir-tblgen` in your build directory and find the
 `mlir-tblgen` binary in the `bin/` subdirectory. All the supported generators