docs: clarify requirements for groupings and measures in AggregateRel (…

…#525)

proto/substrait/algebra.proto

@@ -205,10 +205,12 @@ message AggregateRel {
  // Input of the aggregation
  Rel input = 2;
 
- // A list of expression grouping that the aggregation measured should be calculated for.
+ // A list of one or more grouping expression sets that the aggregation measures should be calculated for.
+ // Required if there are no measures.
  repeated Grouping groupings = 3;
 
  // A list of one or more aggregate expressions along with an optional filter.
+ // Required if there are no groupings.
  repeated Measure measures = 4;
 
  substrait.extensions.AdvancedExtension advanced_extension = 10;