javadoc fixups for com.badlogic.gdx

Add details about other backends, fixup some spelling errors, remove stale documentation from InputProcessor, Graphics, and Net.

Most of the changes are really simple, but in a couple places I expanded the doc from "this on Android, that on Desktop" to a longer list (including GWT, and iOS).  I'm not sure if you'd like this to happen this way, or if the documentation in this platform independent code should just be more vague about the platform specifics?  (E.g., just say "some platforms may not have touch, and will support a mouse." as opposed to enumerating the backend specifics?)

Also, I noticed that the GWT and iOS documentation isn't published with the nightlies.  Is that intentional or on a todo list already?  Is it okay to create {@link} references from the generic code into these backends?

The github diff viewer doesn't seem to be wide enough to accommodate the javadoc, which makes the diff annoying to review.  Sorry about that.  Reformatting the javadoc would probably be more annoying.... :)