Add a picture to show what an option does.
Merge pull request #214 from bangerth/extend-documentation

Add a picture to show what an option does.
Fix a typo.
Merge pull request #215 from bangerth/extend-documentation-2

Fix a typo.
adjust a default value to account for the use of a different temperature in the original source of the melting model
Merge pull request #216 from jdannberg/melt_fraction_fix

adjust a default value to account for the use of a different temperature in the original source of the melting model.

I just noticed that in the Sobolev et al (2011) paper they use the potential temperature instead of the absolute temperature. This means they do not consider the adiabatic temperature change and their melting temperature is lower compared to the one that should be used in a model with the "real" temperature.