gh-132416: document behavior of Path.symlink_to() when link or/and target already exist

Author: alex-semenyuk

Doc/library/pathlib.rst

@@ -1535,6 +1535,8 @@ Creating files and directories
    as a directory if *target_is_directory* is true or a file symlink (the
    default) otherwise.  On non-Windows platforms, *target_is_directory* is ignored.
 
+   If the link or/and the target already exist, :exc:`FileExistsError` is raised.
+
    ::
 
       >>> p = Path('mylink')