Minor: improve RowFilter and ArrowPredicate docs (#6301)

* Minor: improve `RowFilter` and `ArrowPredicate` docs

* tweak

parquet/src/arrow/arrow_reader/filter.rs

@@ -20,6 +20,8 @@ use arrow_array::{BooleanArray, RecordBatch};
 use arrow_schema::ArrowError;
 
 /// A predicate operating on [`RecordBatch`]
+///
+/// See [`RowFilter`] for more information on the use of this trait.
 pub trait ArrowPredicate: Send + 'static {
     /// Returns the [`ProjectionMask`] that describes the columns required
     /// to evaluate this predicate. All projected columns will be provided in the `batch`
@@ -29,7 +31,7 @@ pub trait ArrowPredicate: Send + 'static {
     /// Evaluate this predicate for the given [`RecordBatch`] containing the columns
     /// identified by [`Self::projection`]
     ///
-    /// Must return a  [`BooleanArray`] that has the same length as the input
+    /// Must return a [`BooleanArray`] that has the same length as the input
     /// `batch` where each row indicates whether the row should be returned:
     /// * `true`:the row should be returned
     /// * `false` or `null`: the row should not be returned
@@ -68,12 +70,17 @@ where
     }
 }
 
-/// A [`RowFilter`] allows pushing down a filter predicate to skip IO and decode
+/// Filter applied *during* the parquet read process
+///
+/// [`RowFilter`] applies predicates in order, after decoding only the columns
+/// required. As predicates eliminate rows, fewer rows from subsequent columns
+/// may be required, thus potentially reducing IO and decode.
 ///
-/// This consists of a list of [`ArrowPredicate`] where only the rows that satisfy all
-/// of the predicates will be returned. Any [`RowSelection`] will be applied prior
+/// A `RowFilter` consists of a list of [`ArrowPredicate`]s. Only the rows for which
+/// all the predicates evaluate to `true` will be returned.
+/// Any [`RowSelection`] provided to the reader will be applied prior
 /// to the first predicate, and each predicate in turn will then be used to compute
-/// a more refined [`RowSelection`] to use when evaluating the subsequent predicates.
+/// a more refined [`RowSelection`] used when evaluating the subsequent predicates.
 ///
 /// Once all predicates have been evaluated, the final [`RowSelection`] is applied
 /// to the top-level [`ProjectionMask`] to produce the final output [`RecordBatch`].