Further small improvements in light deflection calculations

In #10666, the light deflection calculation was updated to ensure that a source was not simply taken at infinite distance. In principle, as noted by @mkbrewer in https://github.com/astropy/astropy/pull/10666#issuecomment-678528759 (and following) and https://github.com/astropy/astropy/pull/10666#issuecomment-707369857, two further improvements are possible:
1. One should include that the deflecting source moved a bit in the time the light took to travel to the observer.
2. Other bodies than the Sun also lead to some light deflection (Jupiter at least should be easy to include with `ldn`); see https://github.com/astropy/astropy/pull/10666#issuecomment-707827131 for a plot with effects of the Earth.

Separately, we should document what exactly we do and do not include.

cc @mkbrewer, @StuartLittlefair

Start by reading issue #10666 and the linked pull-request comments, especially the discussions about source motion and additional deflecting bodies. Clarify the intended calculation changes and documentation scope first; done means the agreed effects are implemented and the included and omitted physics are documented.

Written by the indexing model from the issue text.