parser: add VIRTUAL syntax to help

Release note: None

pkg/sql/parser/sql.y

@@ -6163,7 +6163,7 @@ alter_schema_stmt:
 //   FAMILY <familyname>, CREATE [IF NOT EXISTS] FAMILY [<familyname>]
 //   REFERENCES <tablename> [( <colnames...> )] [ON DELETE {NO ACTION | RESTRICT}] [ON UPDATE {NO ACTION | RESTRICT}]
 //   COLLATE <collationname>
-//   AS ( <expr> ) STORED
+//   AS ( <expr> ) { STORED | VIRTUAL }
 //
 // Interleave clause:
 //    INTERLEAVE IN PARENT <tablename> ( <colnames...> ) [CASCADE | RESTRICT]