docs: reflect changes in textual specification/web site

Adapt the textual specification of `AgregateRel` in the specification,
which appears on the web site. In particular:

* Introduce the list of grouping expressions and define grouping sets as
  references into that list.
* Specify that each grouping expression must be referred to by at least
  one grouping set.
* Remove the mentioning of expression equality, which is now not needed
  anymore.

The commit also makes two non-semantic changes in the same paragraphs
that aim to clarify the text:

* Rephrase the explanation on when two records are folded into the same
  group to be more clear.
* Mention explicitly that one empty grouping set is equivalent to not
  having any grouping set.

Signed-off-by: Ingo Müller <[email protected]>

site/docs/relations/logical_relations.md

@@ -345,13 +345,13 @@ The aggregate operation groups input data on one or more sets of grouping keys,
 | Inputs               | 1                                                            |
 | Outputs              | 1                                                            |
 | Property Maintenance | Maintains distribution if all distribution fields are contained in every grouping set. No orderedness guaranteed. |
-| Direct Output Order  | The list of distinct columns from each grouping set (ordered by their first appearance) followed by the list of measures in declaration order, followed by an `i32` describing the associated particular grouping set the value is derived from (if applicable). |
+| Direct Output Order  | The list of grouping expressions in declaration order followed by the list of measures in declaration order, followed by an `i32` describing the associated particular grouping set the value is derived from (if applicable). |
 
 In its simplest form, an aggregation has only measures. In this case, all records are folded into one, and a column is returned for each aggregate expression in the measures list.
 
-Grouping sets can be used for finer-grained control over which records are folded. Within a grouping set, two records will be folded together if and only if each expressions in the grouping set yields the same value for each. The values returned by the grouping sets will be returned as columns to the left of the columns for the aggregate expressions. If a grouping set contains no grouping expressions, all rows will be folded for that grouping set.
+Grouping sets can be used for finer-grained control over which records are folded. A grouping set consists of zero or more references to the list of grouping expressions. Within a grouping set, two records will be folded together if and only if they have the same values for each of the expressions in the grouping set. The values returned by the grouping expressions will be returned as columns to the left of the columns for the aggregate expressions. Each of the grouping expressions must occur in at least one of the grouping sets. If a grouping set contains no grouping expressions, all rows will be folded for that grouping set. (Having a single grouping set with no grouping expressions is thus equivalent to not haveing any grouping sets.)
 
-It's possible to specify multiple grouping sets in a single aggregate operation. The grouping sets behave more or less independently, with each returned record belonging to one of the grouping sets. The values for the grouping expression columns that are not part of the grouping set for a particular record will be set to null. Two grouping expressions will be returned using the same column if the protobuf messages describing the expressions are equal. The columns for grouping expressions that do *not* appear in *all* grouping sets will be nullable (regardless of the nullability of the type returned by the grouping expression) to accomodate the null insertion.
+It is possible to specify multiple grouping sets in a single aggregate operation. The grouping sets behave more or less independently, with each returned record belonging to one of the grouping sets. The values for the grouping expression columns that are not part of the grouping set for a particular record will be set to null. The columns for grouping expressions that do *not* appear in *all* grouping sets will be nullable (regardless of the nullability of the type returned by the grouping expression) to accomodate the null insertion.
 
 To further disambiguate which record belongs to which grouping set, an aggregate relation with more than one grouping set receives an extra `i32` column on the right-hand side. The value of this field will be the zero-based index of the grouping set that yielded the record.