doc: default model_mod routines are used

[hkershaw-brown]

Incorrect documentation

Default model_mod, stays routines do no work - not true.

https://docs.dart.ucar.edu/en/latest/models/utilities/default_model_mod.html

Side note: why does default_model have all the model_mod routines, rather than just the default ones you can use? Left over from default being used as template_model_mod.f90, but now we have a template?