Make sure precipitateEvolution documentation matches the code

[stvdwtt]

Currently, the documentation gives the standard 3rd order interpolation function but the code actually uses the standard 5th order interpolation function.

Also, the documentation includes the Landau term even though the code doesn't have it.

The documentation and the code should match. (Note this could have knock-on effects on problem 7 here: https://github.com/prisms-center/PRISMS-PF_Training_Materials/blob/master/PRISMS-PF%20Exercises.pdf)

[david-montiel-t]

Documentation has been fixed.