Many free software projects are satisfied with releasing code. Some think that they're going above and beyond by including a help function in the program itself, available by typing some command-line flag. Others really go all out and provide a grammatically incorrect and technically vague manual page.
OpenBSD's documentation is expected to be both complete and accurate. The manual pages for system and library calls are extensive, even when compared to the other BSDs, and include discussions on usage and security. In its audit of the OpenBSD source code tree, the OpenBSD team found any number of circumstances where people had used the library interface as the manual page said they should, but the manual page was incorrect! This created both potential and actual security problems. As such, a documentation error is considered a serious bug and treated as harshly as any other serious bug.