Improve documentation for volume fraction of hollow models

[pkienzle]

sasview #2794 shows that confusion persists.

Including Ryan's example of core-shell sphere vs. vesicle model in the docs would help.

We could also modify the description of volfraction for vesicle to say that it is uses the volume fraction of the enclosed volume when computing the structure factor. (See also sasview #3973)

Originally posted by @pkienzle in #232