document that git.cached_repository=true is needed for searching in git changesets

[anonymous]

I was testing the search in the commit messages of the git repositories that I added to trac.

I noted that it dind't work until I set git.cached_repository=true in trac.ini, followed by trac-admin /path/to/my/trac/env repository resync "myrepo".

(By the way, when is this trac-admin command necessary? Assuming that the git hooks are configured correctly, is this necessary only once?)

I couldn't find this piece of information in the docs (at least not in TracGit) and thought I could add a reminder here.