Describe option to specify PreParser explicitly

[jfingerh]

This was discussed a fair amount around Jan 2021, and there are some notes on at least some ideas on how this could be done from that time in the meeting notes from then.

[jafingerhut]

Closing this issue without any further action, as it is only relevant for the same reasons that the 2-parser vs. 1-parser PNA variant were, which is also being abandoned, as for this PR: #51