Bug 571455 - Better document disposal of images

Fix DisposeListener link. Detected by
https://download.eclipse.org/eclipse/downloads/drops4/I20210311-1800/testresults/html/org.eclipse.ua.tests.doc_ep420I-unit-cen64-gtk3-java11_linux.gtk.x86_64_11.html

Change-Id: I18b5af0d07ea5524f5823f4e585c2908885d9851
Signed-off-by: Alexander Kurtakov <akurtako@redhat.com>