INFO: ds9 marker parameters

This is part of a larger effort to collect what parameters do different backends take for markers. For ds9 the concept of "markers" is not cleanly separated from "regions" - in ds9 they are the *same thing*. But it's still useful to highlight the regions that are actually "marker-like". The authoritative source I think is actually in the ds9 docs for what ever version you're running, but I think http://ds9.si.edu/doc/ref/region.html is the most authoritative online reference.

The main regions most relevant for markers in my experience are `point`, `circle`, and `ellipse`. The format for the region file for those are below:

```
point x y # point=[circle|box|diamond|cross|x|arrow|boxcircle] [size]
circle x y radius # fill=[0|1]
ellipse x y radius radius angle# fill=[0|1]
```
each of those can take a much longer list of properties that are listed at http://ds9.si.edu/doc/ref/region.html#RegionProperties I think the most relevant ones for markers, though, are `color`, `width`, `text`, `font`.

That pretty much covers it, although with the caveat that any arbitrary sort of region *might* be used as a "marker" - e.g. a weird star-shaped polygon could be used as a "star" marker.

Start with the referenced DS9 region documentation, especially the point, circle, and ellipse formats and the listed properties such as color, width, text, and font. Determine where this backend marker information belongs in the project; done means the relevant DS9 marker-like region parameters are documented with the noted caveat that other regions may also be used.

Written by the indexing model from the issue text.