> Five years and 80% employee turnover from now, your git docs will be a completely unsearchable pile of outdated trash from employees who don't work there anymore

Throwing your own other comment back at ya :D

If docs aren't maintained and curated then it doesn't matter if they are in a wiki, word docs, printed pages in a file cabinet or in the repository as markdown files. Getting team discipline around updating docs is the solution, the place where they are stored is sort of trivial if that discipline exists (fwiw, I've yet to see that discipline).