Bug#849419: libopenjfx-java: Description should clarify libopenjfx-java is not automatically on classpath without pkg openjfx

[Emmanuel Bourg]

Depending on openjfx instead of libopenjfx-java is the intended usage of
the package. libopenjfx-java contains only the library, and openjfx
integrates it with the default JRE. This split was necessary to optimize
the size of the package (openjfx is arch any, if it contained the JavaFX
jars it would waste a lot of space on the mirrors).

Emmanuel Bourg