message: Update documentation for the DialectOpconversion class: overriding `match()` is optional
  Recently a default implementation for `match()` was provided (cl/242285885), but the class documentation wasn't updated appropriately.
diff: @@ -42,8 +42,10 @@ class FunctionConversion;
  }
  /// Base class for the dialect op conversion patterns. Specific conversions
  -/// must derive this class and implement `PatternMatch match(Operation *)`
  -/// defined in `Pattern` and at least one of `rewrite` and `rewriteTerminator`.
  +/// must derive this class and implement least one of `rewrite` and
  +/// `rewriteTerminator`. Optionally they can also override
  +/// `PatternMatch match(Operation *)` to match more specific operations than the
  +/// `rootName` provided in the constructor.
  //
  // TODO(zinenko): this should eventually converge with RewritePattern. So far,
  // rewritePattern is missing support for operations with successors as well as
  