Minor: improve GroupsAccumulator docs

datafusion/expr-common/src/groups_accumulator.rs

@@ -56,9 +56,32 @@ impl EmitTo {
  }
 }
 
-/// `GroupAccumulator` implements a single aggregate (e.g. AVG) and
+/// `GroupsAccumulator` implements a single aggregate (e.g. AVG) and
 /// stores the state for *all* groups internally.
 ///
+/// Logically, a [`GroupsAccumulator`] stores a mapping from each group index to
+/// the state of the aggregate for that group. For example an implementation for
+/// `min` might look like
+///
+/// ```text
+/// ┌─────┐
+/// │ 0 │───────────▶ 100
+/// ├─────┤
+/// │ 1 │───────────▶ 200
+/// └─────┘
+/// ... ...
+/// ┌─────┐
+/// │ N-2 │───────────▶ 50
+/// ├─────┤
+/// │ N-1 │───────────▶ 200
+/// └─────┘
+///
+///
+/// Logical group Current Min
+/// number value for that
+/// group
+/// ```
+///
 /// # Notes on Implementing `GroupAccumulator`
 ///
 /// All aggregates must first implement the simpler [`Accumulator`] trait, which