Plasmapy: Add narrative documentation on @validate_quantities to development guide

[namurphy]

The validators created in #648 provide useful checks on arguments and returned values of functions and methods. While the @validate_quantities decorator has a well-written docstring, narrative documentation in the development guide would help put it in context for newcomers and help them understand why and how it is used.

[namurphy]

A possibility would be to have a section on the development guide that focuses on decorators for formulary-like functions. It might be useful to discuss @particle_input in there as well as @validate_quantities, although it would still be useful to at least briefly cover @particle_input in the narrative documentation in plasmapy.particles so people know it exists.

[StanczakDominik]

This would be great as an example (ekhm, notebook ;) ). That could then be linked from the appropriate docs.

[namurphy]

Another thought is that it would be helpful to have an example of using validate_quantities with a setter in the docs, since I got confused about which decorator should go first. An example that could be expanded on would be:

class A: 

    @property 
    def x(self): 
        return 1 

    @x.setter 
    @validate_quantities  # though actually set some actual validations
    def x(self, y): 
        self.y = y 

The @x.setter decorator should go on the outside.