Discoverability is at least as important as the documentation content itself. A lot of really good advice here is given with regards to the content. Obviously,

Discoverability is at least as important as the documentation content itself. A lot of really good advice here is given with regards to the content. Obviously, this is very important; however, it is not enough, when there are multiple places of documentation. Keeping at most 2 (code + wiki or something) sources is essential to ensuring that docs are actually read and can be found by others. In my current company (~3k developers) there are more than 10 different places for documentation. This both makes it difficult to find anything AND discourages developers to write any docs in the first place.