Improve documentation for make_rgb_lupton

I tried using the ``make_rgb_lupton`` on a set of 2MASS images for the Galactic center:

```
import numpy as np
from astropy.visualization import make_lupton_rgb
from astropy.utils.data import get_pkg_data_filename
from astropy.io import fits

image_r = fits.getdata(get_pkg_data_filename('galactic_center/gc_2mass_k.fits'))
image_g = fits.getdata(get_pkg_data_filename('galactic_center/gc_2mass_h.fits'))
image_b = fits.getdata(get_pkg_data_filename('galactic_center/gc_2mass_j.fits'))

image = make_lupton_rgb(image_r, image_g, image_b, filename='galactic_center_default.png')
```

and get the following result:


That's... a lot of yellow....

I cannot figure out how to make this look like a normal star field, and the documentation doesn't explain how to tune the parameters to get a good result. It would be nice to have more extensive documentation to show the effect different parameters have.

@parejkoj - on a shorter timescale, can you explain how to optimize the example above so that I have a better idea how this work?

Where does the algorithm perform best - not so dense star fields?

Is the issue the minimum values and if so should we have better defaults? (for example percentile-based)

Start from the make_lupton_rgb example in the issue and inspect its documented parameters and current output behavior. Done means the documentation explains how parameter choices affect the rendered image, gives guidance for tuning the supplied 2MASS example, and addresses where the algorithm works best.

Written by the indexing model from the issue text.