document pr landing patterns

[ModProg]

Even though any PR targets a single branch, we may be interrested in
learning whether it has landed in any number of branches.
These are a comma separated list of `<target>:<channels>`.
<target> is a regex pattern matching target branches for PRs.
<channels> are a space separated list of glob patterns matching git branches.
For each PR that matches <target>, we check if that PR has landed in each of the
corresponding channels for that <target>.
>>> TODO: explain .flat_map(|(b, c)| c.iter().map(|chan| b.replace(base, chan).to_string())) <<<