An example to need to explain when the user might want to manually set placer ids

[lukasturcani]

Notably in the case of something small like methane, where the functaional groups overlap so the default placer ids are actually the same atom.
The user in this case must set the placers for each fg to be a different atom

[andrewtarzia]

I have attempted this in #517 but will note that it is likely not necessary anymore as other completed issues have solved this problem.