Minor style/documentation changes

Author: nex3

lib/src/value/function.dart

@@ -24,8 +24,10 @@ class SassFunction extends Value {
   /// synchronous evaluate visitor will crash if this isn't a [Callable].
   final AsyncCallable callable;
 
-  /// The unique compile context for tracking if SassFunction and SassMixin
-  /// belongs to current compilation or not.
+  /// The unique compile context for tracking if this [SassFunction] belongs to
+  /// the current compilation or not.
+  ///
+  /// This is `null` for functions defined in plugins' Dart code.
   final Object? _compileContext;
 
   SassFunction(this.callable) : _compileContext = null;

lib/src/value/mixin.dart

@@ -26,8 +26,8 @@ final class SassMixin extends Value {
   @internal
   final AsyncCallable callable;
 
-  /// The unique compile context for tracking if SassFunction and SassMixin
-  /// belongs to current compilation or not.
+  /// The unique compile context for tracking if this [SassMixin] belongs to the
+  /// current compilation or not.
   final Object? _compileContext;
 
   SassMixin(this.callable) : _compileContext = null;

lib/src/visitor/async_evaluate.dart

@@ -136,10 +136,6 @@ final class _EvaluateVisitor
         StatementVisitor<Future<Value?>>,
         ExpressionVisitor<Future<Value>>,
         CssVisitor<Future<void>> {
-  /// The unique compile context for tracking if SassFunction and SassMixin
-  /// belongs to current compilation or not.
-  final Object _compileContext = Object();
-
   /// The import cache used to import other stylesheets.
   final AsyncImportCache? _importCache;
 
@@ -183,6 +179,10 @@ final class _EvaluateVisitor
   /// Whether to track source map information.
   final bool _sourceMap;
 
+  /// The unique compile context for tracking if [SassFunction]s and
+  /// [SassMixin]s belongs to the current compilation or not.
+  final Object _compileContext = Object();
+
   /// The current lexical environment.
   AsyncEnvironment _environment;