Update

Author: alamb

datafusion/expr/src/udaf.rs

@@ -651,11 +651,13 @@ pub trait AggregateUDFImpl: Debug + DynEq + DynHash + Send + Sync {
         AggregateOrderSensitivity::HardRequirement
     }
 
-    /// Return a closure for simplifying a user defined aggregate.
+    /// Return a closure for simplifying a user-defined aggregate.
     ///
-    /// Use this to apply function specific simplification rules during
-    /// optimization (e.g. `percentile_cont()` --> `Min`). The default
-    /// implementation does nothing.
+    /// Use this to apply function-specific simplification rules during
+    /// optimization.  The default implementation does nothing.
+    ///
+    /// For example, `percentile_cont()` can be replaced `MIN` or `MAX` when the
+    /// target percentile is `0.0` or `1.0`, respectively).
     ///
     /// Note that DataFusion handles simplifying arguments and  "constant
     /// folding" (replacing a function call with constant arguments such as
@@ -667,7 +669,7 @@ pub trait AggregateUDFImpl: Debug + DynEq + DynHash + Send + Sync {
     /// [None] if simplify is not defined
     ///
     /// Or, a closure with two arguments:
-    /// * 'aggregate_function': [AggregateFunction], which includes already simplified
+    /// * 'aggregate_function': [AggregateFunction], which already includes simplified
     ///   arguments
     /// * 'info': [crate::simplify::SimplifyContext]
     ///