Here is a list of projects whose documentation I found to be highly-detailed and well-written.

big

• The introductory tutorial of GNU Emacs (looks a lot better when run from within Emacs), is written in a very gentle and careful way; it's among the best tutorials I've seen. Same applies to the Emacs manual, which I'm tempted to call the best I've read yet. It really, really goes into painstaking detail, and seems to have been written by professional technical writers.
• CPython - this is quite massive, so although quality is generally high, it varies depending on where you are; an example of excellence
• sqlite3 - quite extensive, especially for an embeddable DBMS

small

• Suds - multiple, well-written examples
• rdiff-backup usage examples - much smaller than the others listed here, but nevertheless deserves mention because it's both gentle and thorough
• While working on giving a product of ours at work the ability to speak SocketCAN, I really appreciated its official documentation. It is very well-written and thorough (though it assumes prior knowledge of working with Linux sockets).