When you need to explain a long thing you have to rely to multiple posts, and sometimes such long explanations are needed for the other party understand your view on the matter.
sources:
Daesh discussion I wrote ~1000 characters in two posts (And source) in order to make my point
But the very point of the argument map is to write short and concise premises. If there are any long explanations, those would be provided in sources such as Wikipedia. Such sources can be linked. So there is no need to include long explanations.
An explanation itself consists of justification for the claim; which means it can be broken up into premises and claim. So, rather than writing an explanation, just add those subnodes.
Sometimes it is necessary to describe certain points of a conversation in greater detail because some points can be easily misinterpreted or misunderstood.