doc: update fs flags documentation

PR-URL: #48463
Fixes: #48452
Reviewed-By: Antoine du Hamel <duhamelantoine1995@gmail.com>
Reviewed-By: LiviaMedeiros <livia@cirno.name>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Mestery <mestery@protonmail.com>
Reviewed-By: Mohammed Keyvanzadeh <mohammadkeyvanzade94@gmail.com>

Author: sinkhaha

doc/api/fs.md

@@ -7877,6 +7877,9 @@ string.
 * `'r'`: Open file for reading.
   An exception occurs if the file does not exist.
 
+* `'rs'`: Open file for reading in synchronous mode.
+  An exception occurs if the file does not exist.
+
 * `'r+'`: Open file for reading and writing.
   An exception occurs if the file does not exist.