chore: update docs links.

Author: wiedld

datafusion/expr/src/expr_schema.rs

@@ -404,9 +404,9 @@ impl ExprSchemable for Expr {
     /// - **Binary expressions**: field metadata is empty
     /// - **Boolean expressions**: field metadata is empty
     /// - **Cast expressions**: determined by the input expression's field metadata handling
-    /// - **Scalar functions**: Generate metadata via function's `return_field_from_args` method,
+    /// - **Scalar functions**: Generate metadata via function's [`return_field_from_args`] method,
     ///   with the default implementation returning empty field metadata
-    /// - **Aggregate functions**: Generate metadata via function's `return_field` method,
+    /// - **Aggregate functions**: Generate metadata via function's [`return_field`] method,
     ///   with the default implementation returning empty field metadata
     /// - **Window functions**: field metadata is empty
     ///
@@ -416,6 +416,9 @@ impl ExprSchemable for Expr {
     ///
     /// So for example, a projected expression `col(c1) + col(c2)` is
     /// placed in an output field **named** col("c1 + c2")
+    ///
+    /// [`return_field_from_args`]: crate::ScalarUDF::return_field_from_args
+    /// [`return_field`]: crate::AggregateUDF::return_field
     fn to_field(
         &self,
         schema: &dyn ExprSchema,

datafusion/expr/src/logical_plan/plan.rs

@@ -2198,8 +2198,8 @@ impl Projection {
 /// # Metadata Handling
 ///
 /// - **Schema-level metadata**: Passed through unchanged from the input schema
-/// - **Field-level metadata**: Determined by each expression via `exprlist_to_fields`, which
-///   calls `Expr::to_field` to handle expression-specific metadata (literals, aliases, etc.)
+/// - **Field-level metadata**: Determined by each expression via [`exprlist_to_fields`], which
+///   calls [`Expr::to_field`] to handle expression-specific metadata (literals, aliases, etc.)
 ///
 /// # Returns
 ///

datafusion/expr/src/utils.rs

@@ -699,7 +699,7 @@ where
 /// - **Metadata**: Computed based on expression type (preserving, merging, or generating new metadata)
 /// - **Table reference scoping**: Establishing proper qualified field references
 ///
-/// Each expression is converted to a field by calling `Expr::to_field`, which performs
+/// Each expression is converted to a field by calling [`Expr::to_field`], which performs
 /// the complete field resolution process for all field properties.
 ///
 /// # Returns