True, but if you developed a mod that far that documenting everything yourself becomes problematic you most likely already aquired a solid fanbase that takes care of such stuff. In case of TE there are a lot of vids explaining his mod and of course the FTB-Wiki (well there seems to be 2 FTB-Wikis^^). But then again TE is pretty popular and if you want to know something about the mod you normally get the informations.
The Wiki lags a lot of information like capacity for Tesseracts or Energy-Conduits, Information about the Cyclic-Assembler (although there is a unfinished page) and the Insertion Pipe and some Pages like the one for the Igneous Extruder which is simply wrong, since it states that Stone takes 2mb Lava and 500mb Water (it takes no Lava and 1000mb Water). But at this point the mod develops really fast and you have to decide what to do. Will you push back content and fix all the documentation first? Will you propably set a new Milestone and make a new documentation after your done? Should you evt. try to get somebody who only does the documentation and nothing else? Should you evt. just ignore documentation and just provide status-updates and changelogs via the page?
I cant really believe that documenting your mod is something fun to do. First you made the damn thing. You might take things as given others do not. So explaining something can be tricky. Second proper documentation is basically just making things up and write them down. It takes a lot of time. It also is a public document written by you, so you want to make it shine for you, since a lot of modders work as programmers, and a good mod can be a nice bonus for your carreer, but in that case if you do something you have to do it properly.
I doubt that if someone approaches lemming and asks him if he would allow him to do a proper documentation for TE he would turn that down. He could even invest some time to ask some questions for clarification and help with it (although properly under his condition, like placing the documentation in the FTB-Wiki or something, which misses informations on TE). I guess in such cases, if done properly they gladly take others aid. It is a lot less work to look through someone else documentation and evt. fixing minor issues than having to do it all by yourself. Exspecially considering that the person who made the documentation learned the mod in the first place too, which helps providing valuable informations.