I think that very few people would argue that there should not be such documentation.
The real problem, faced by every application in existence, is “who is going to write it and curate the documentation?”
Generally speaking, developers of open source/community apps prefer to spend their time writing features and fixing bugs to writing documentation. But they are really the only people who can authoritatively say what the application does.
It’s not impossible to have community developed and curated content of this nature.
Knowledge about how things work can be captured from posts from the developers, just like BHyden did up there, and put in one place.
Do you have a proposal for how and where this should be done, and by whom?