message: [DRR] List some limitations clearly in the doc
diff: @@ -43,8 +43,16 @@ operations. This gives DRR both its strengths and limitations: it is good at
  expressing op to op conversions, but not that well suited for, say, converting
  an op into a loop nest.
  -Per the current implementation, DRR also does not have good support for regions
  -in general.
  +Per the current implementation, DRR does not have good support for the following
  +features:
  +
  +* Matching and generating ops with regions.
  +* Matching and generating ops with block arguments.
  +* Matching multi-result ops in nested patterns.
  +* Matching and generating variadic operand/result ops in nested patterns.
  +* Packing and unpacking variaidc operands/results during generation.
  +* [`NativeCodeCall`](#native-code-call-transforming-the-generated-op)
  + returning more than one results.
  ## Rule Definition
  @@ -345,6 +353,9 @@ template. The string can be an arbitrary C++ expression that evaluates into
  some C++ object expected at the `NativeCodeCall` site (here it would be
  expecting an array attribute). Typically the string should be a function call.
  +Note that currently `NativeCodeCall` must return no more than one value or
  +attribute. This might change in the future.
  +
  ##### `NativeCodeCall` placeholders
  In `NativeCodeCall`, we can use placeholders like `$_builder`, `$N`. The former
  