Improve documentation of delegation rules

Improve documentation of delegation rules

Per feedback from FOSDEM, this is still confusing some people. Clarify
things.

You might think we could just strip of the offending prefixes but that
might not always be the thing to do. Other VCSs don't include these
prefixes and both 'a' and 'b' are valid folder names. The risk of false
positives might be small, but it's enough to discourage us from doing
this.

Signed-off-by: Stephen Finucane <stephen@that.guru>
Reviewed-by: Thomas Monjalon <thomas.monjalon@6wind.com>