Finish documentation, remove unnecessary null hint

sqlparser/lib/src/ast/statements/invalid.dart

@@ -1,6 +1,6 @@
 part of '../ast.dart';
 
-/// Used as a top-level parsing
+/// Used as a top-level substitute when no statement could be parsed otherwise.
 class InvalidStatement extends Statement {
   @override
   R accept<A, R>(AstVisitor<A, R> visitor, A arg) {

sqlparser/lib/src/reader/parser/parser.dart

@@ -226,7 +226,7 @@ class Parser extends ParserBase
   var _lastStmtHadParsingError = false;
 
   /// Parses a statement without throwing when there's a parsing error.
-  Statement /*?*/ safeStatement() {
+  Statement safeStatement() {
     return _parseAsStatement(statement, requireSemicolon: false) ??
         InvalidStatement();
   }