Improve ProjectionExpr documentation and comments (#19263)

## Which issue does this PR close?

- related to #19111

## Rationale for this change

While reviewing #19111 from
@adriangb I found a few places that the documentation could be improved
to give higher level context

## What changes are included in this PR?

Update some docs and comments

## Are these changes tested?

By cos

## Are there any user-facing changes?
Just docs/comments. No functional changes

Author: alamb

datafusion/datasource-parquet/src/opener.rs

@@ -286,8 +286,9 @@ impl FileOpener for ParquetOpener {
             // - The table schema as defined by the TableProvider.
             //   This is what the user sees, what they get when they `SELECT * FROM table`, etc.
             // - The logical file schema: this is the table schema minus any hive partition columns and projections.
-            //   This is what the physicalfile schema is coerced to.
-            // - The physical file schema: this is the schema as defined by the parquet file. This is what the parquet file actually contains.
+            //   This is what the physical file schema is coerced to.
+            // - The physical file schema: this is the schema that the arrow-rs
+            //   parquet reader will actually produce.
             let mut physical_file_schema = Arc::clone(reader_metadata.schema());
 
             // The schema loaded from the file may not be the same as the

datafusion/physical-expr/src/projection.rs

@@ -15,6 +15,8 @@
 // specific language governing permissions and limitations
 // under the License.
 
+//! [`ProjectionExpr`] and [`ProjectionExprs`] for representing projections.
+
 use std::ops::Deref;
 use std::sync::Arc;
 
@@ -35,14 +37,16 @@ use datafusion_physical_expr_common::utils::evaluate_expressions_to_arrays;
 use indexmap::IndexMap;
 use itertools::Itertools;
 
-/// A projection expression as used by projection operations.
+/// An expression used by projection operations.
 ///
 /// The expression is evaluated and the result is stored in a column
 /// with the name specified by `alias`.
 ///
 /// For example, the SQL expression `a + b AS sum_ab` would be represented
 /// as a `ProjectionExpr` where `expr` is the expression `a + b`
 /// and `alias` is the string `sum_ab`.
+///
+/// See [`ProjectionExprs`] for a collection of projection expressions.
 #[derive(Debug, Clone)]
 pub struct ProjectionExpr {
     /// The expression that will be evaluated.
@@ -107,11 +111,14 @@ impl From<ProjectionExpr> for (Arc<dyn PhysicalExpr>, String) {
     }
 }
 
-/// A collection of projection expressions.
+/// A collection of  [`ProjectionExpr`] instances, representing a complete
+/// projection operation.
+///
+/// Projection operations are used in query plans to select specific columns or
+/// compute new columns based on existing ones.
 ///
-/// This struct encapsulates multiple `ProjectionExpr` instances,
-/// representing a complete projection operation and provides
-/// methods to manipulate and analyze the projection as a whole.
+/// See [`ProjectionExprs::from_indices`] to select a subset of columns by
+/// indices.
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct ProjectionExprs {
     exprs: Vec<ProjectionExpr>,