In my opinion it would be fine to do a release with partial documentation just to get some feedback. For some modules it may be enough to start with a general description on what the module is for and how to use it, leaving the details to the 'hints' already present in most UI elements. In those cases you could simply add a paragraph stating "This section will be updated in a future release. For now please refer to mouse-over hints provided by the dialog.".
"Partial documentation" is already being made available through these posts for anyone to give feedback.
I would only make an official release once the documentation is complete, or in this case, once all modules are covered.
No offense, but people pay for this, and while PixInsight is a constantly evolving project, I would avoid giving people the feeling they paid for an incomplete product.
Incomplete documentation gives that feeling and it doesn't look professional.
My 2 c's...