Update SystemGenerator to reflect being able to pass a list for SMIRNOFFTemplateGenerator

[IAlibay]

With #423 we can now pass an iterable to SMIRNOFFTemplateGenerator, allowing users to define multiple offxmls.

The current SystemGenerator docstring does not currently reflect this:

openmmforcefields/openmmforcefields/generators/system_generators.py

Line 90 in 0fdae52

small_molecule_forcefield : str, optional, default='openff-2.2.0'

At the minimum, it may be good to update this docstring. I also wonder if we need some more type checking in the GAFF and Espaloma template generators so that they can catch a case where someone passed in an iterable intsead of a string.

[epretti]

I've just opened #435 to update this and some other outdated documentation.

I think I also see a problem where an iterable can make it into EspalomaTemplateGenerator and raise a TypeError instead of a ValueError which breaks SystemGenerator's attempt to screen out template generators that don't support a given force field input. I'll fix that separately.

[epretti]

#435 and #438 are now merged, so this should be fixed.