I suggest that instead of a wiki, the documentation gets into the main repo in a "documentation" folder, as .mediawiki and/or as .md and/as .adoc files. together with the source code. I think this allows a better synchronization of source vs doc as it's possible to say on a Gerrit patch or PR "Please update file xyz.adoc" at the same time. It also allows to track contributors to documentation as code contributors, reflected in the same Git history, facilitating inclusion of doc related changes in N&N, which is relatively fair.
Since GitHub provides an "edit" button which automatically starts a PR and has a good preview for doc files, it is as simple to contribute to doc in the GitHub repo than it is the contribute directly to a wiki. It also allows to enforce review which ultimately lead to more awareness from committers about what needs to be documented (leading to ideas of improvements) and improve quality of the documentation (preventing erroneous statements in documentation).
Cheers
