Documenting code is the bane of many a programmer's existence. While code documentation is important, some programmers prefer to leave that work to technical writers. Others will provide a bare minimum of information, sometimes as README files or other external documents. Generally speaking, unless a program is supported by a company or organization, homebrew software has just enough information to tell you how to use it.
To be honest, some documentation comes across as being notes from the development timeline, rather than useful documentation. Many authors give up on installing a program because the documentation is inadequate, particularly when troubleshooting a bad install.