Update documenation of drucker prager parse_parameters()

[anne-glerum]

Fixes #4273.

Update the documentation of the parse_parameters function now that its implementation has changed.

@gassmoeller For dislocation and diffusion creep, the documentation says it's the number of phase transitions. The way I read the code, it's the number of phases, so I've updated the documentation accordingly. Is this correct?

For all pull requests:

• I have followed the instructions for indenting my code.

[gassmoeller]

This is correct. I dont know why diffusion and dislocation creep refer to it as the number of transitions, it should be the number of phases (if they use the same parameter).