According to the Wikipidia article:
"A living document or dynamic document is a document which may be continually edited and updated by either a limited or unrestricted group. A simple example of a living document is an article in Wikipedia, in contrast to "dead" or "static" documents, such as an article in a single edition of the Encyclopædia Brittanica. This is sometimes also referred to as an "evergreen document" (always evolving/updating/expanding) as needed."
So our Wiki is one big living document, but sometimes we want to emphasize that something is still under a fair amount of construction or will develop organically as we use it.