[_fe_analyzer_shared] Switch syntatcic_entity.dart to /// comments.

This brings the file into alignment with the style guidelines (see
https://dart.dev/effective-dart/documentation#do-use-doc-comments-to-document-members-and-types).

Change-Id: I6a6a696403c3aaf24cf8ac2b8ba6f684a70d1480
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/467681
Commit-Queue: Paul Berry <paulberry@google.com>
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>

Author: stereotype441

pkg/_fe_analyzer_shared/lib/src/base/syntactic_entity.dart

@@ -4,28 +4,20 @@
 
 import 'package:_fe_analyzer_shared/src/base/analyzer_public_api.dart';
 
-/**
- * Interface representing a syntactic entity (either a token or an AST node)
- * which has a location and extent in the source file.
- */
+/// Interface representing a syntactic entity (either a token or an AST node)
+/// which has a location and extent in the source file.
 @AnalyzerPublicApi(
   message: 'exported by package:analyzer/dart/ast/syntactic_entity.dart',
 )
 abstract class SyntacticEntity {
-  /**
-   * Return the offset from the beginning of the file to the character after the
-   * last character of the syntactic entity.
-   */
+  /// Return the offset from the beginning of the file to the character after
+  /// the last character of the syntactic entity.
   int get end;
 
-  /**
-   * Return the number of characters in the syntactic entity's source range.
-   */
+  /// Return the number of characters in the syntactic entity's source range.
   int get length;
 
-  /**
-   * Return the offset from the beginning of the file to the first character in
-   * the syntactic entity.
-   */
+  /// Return the offset from the beginning of the file to the first character in
+  /// the syntactic entity.
   int get offset;
 }