Minor: Improve ExecutionMode documentation (#12214)

* Minor: Improve ExecutionMode documentation

* Update datafusion/physical-plan/src/execution_plan.rs

Co-authored-by: Mehmet Ozan Kabak <[email protected]>

* Update datafusion/physical-plan/src/execution_plan.rs

Co-authored-by: Mehmet Ozan Kabak <[email protected]>

---------

Co-authored-by: Mehmet Ozan Kabak <[email protected]>

datafusion/physical-plan/src/execution_plan.rs

@@ -482,24 +482,43 @@ impl ExecutionPlanProperties for &dyn ExecutionPlan {
     }
 }
 
-/// Describes the execution mode of an operator's resulting stream with respect
-/// to its size and behavior. There are three possible execution modes: `Bounded`,
-/// `Unbounded` and `PipelineBreaking`.
+/// Describes the execution mode of the result of calling
+/// [`ExecutionPlan::execute`] with respect to its size and behavior.
+///
+/// The mode of the execution plan is determined by the mode of its input
+/// execution plans and the details of the operator itself. For example, a
+/// `FilterExec` operator will have the same execution mode as its input, but a
+/// `SortExec` operator may have a different execution mode than its input,
+/// depending on how the input stream is sorted.
+///
+/// There are three possible execution modes: `Bounded`, `Unbounded` and
+/// `PipelineBreaking`.
 #[derive(Clone, Copy, PartialEq, Debug)]
 pub enum ExecutionMode {
-    /// Represents the mode where generated stream is bounded, e.g. finite.
+    /// The stream is bounded / finite.
+    ///
+    /// In this case the stream will eventually return `None` to indicate that
+    /// there are no more records to process.
     Bounded,
-    /// Represents the mode where generated stream is unbounded, e.g. infinite.
-    /// Even though the operator generates an unbounded stream of results, it
-    /// works with bounded memory and execution can still continue successfully.
+    /// The stream is unbounded / infinite.
     ///
-    /// The stream that results from calling `execute` on an `ExecutionPlan` that is `Unbounded`
-    /// will never be done (return `None`), except in case of error.
+    /// In this case, the stream will never be done (never return `None`),
+    /// except in case of error.
+    ///
+    /// This mode is often used in "Steaming" use cases where data is
+    /// incrementally processed as it arrives.
+    ///
+    /// Note that even though the operator generates an unbounded stream of
+    /// results, it can execute with bounded memory and incrementally produces
+    /// output.
     Unbounded,
-    /// Represents the mode where some of the operator's input stream(s) are
-    /// unbounded; however, the operator cannot generate streaming results from
-    /// these streaming inputs. In this case, the execution mode will be pipeline
-    /// breaking, e.g. the operator requires unbounded memory to generate results.
+    /// Some of the operator's input stream(s) are unbounded, but the operator
+    /// cannot generate streaming results from these streaming inputs.
+    ///
+    /// In this case, the execution mode will be pipeline breaking, e.g. the
+    /// operator requires unbounded memory to generate results. This
+    /// information is used by the planner when performing sanity checks
+    /// on plans processings unbounded data sources.
     PipelineBreaking,
 }