Document how nonbonded methods get resolved

[j-wags]

Is your feature request related to a problem? Please describe.
@SimonBoothroyd asked how openMM nonbonded methods get selected based on user input. We had discussed this for a long time, and come up with a decision table, but it never made it into the documentation. The current decision table is recorded in the tests here:

openff-toolkit/openff/toolkit/tests/test_forcefield.py

Line 595 in 2380061

nonbonded_resolution_matrix = [

Describe the solution you'd like
That table should be in the ReadTheDocs. The best place for it is probably the ForceField docstring.