What can I say: documentation is always a weakness.
From the Solr side, I think most of the documentation tends to be higher level on the wiki, and javadocs
don't get as much attention since its a smaller population hacking on the APIs.
From the Lucene side, things tend to be under-documented or out of date.
In any case, just throw up patches and lets get em in. Nobody gets excited to work on docs too much around here unfortunately, probably because there is no momentum and the existing docs are all out of date and not very good.
On the other hand, just in the past weekend we've made some serious progress on the lucene documentation already: nuked the old out-of-date-forrest stuff, fixed a ton of broken links, added a broken-link checker, linked the javadocs of various modules to each other, brought all the existing docs (minus fileformats) up to speed with 4.0, generating htmlized version of the .txt documents with markdown, etc.
I know there is a lot of underdocumented stuff in lucene, but currently from my own perspective I am working to correct the broken stuff
For some reason, its definitely harder to fix the old documentation up to make sense than i would have ever thought, I spent most of the day just bringing http://lucene.apache.org/core/3_6_0/scoring.html up to speed and integrating it into the o.a.l.search package javadocs.
For the stuff you see thats experimental with no javadoc tag... this is really just as bad of a problem, just open issues if you want to help out. We are pretty overwhelmed with things to fix on the documentation side so any help would be appreciated.