WICKET-5652 Improve Javadoc of Ajax behaviors concerning their onXyz() methods

(cherry picked from commit 56f8c4ca0d0dd52b9d61d48aa1ec699b129d103c)
WICKET-5660 Throw more specific exception when a component cannot be found by AutoLabelTextResolver

(cherry picked from commit de46d5ab95274f9d19c4d372a7f405e705e48384)