Document approach for must-be-supported-or-rejected config items

Configurations with bridges are rejected, if bridge support is disabled, following !877 (merged)

As per !877 (comment 2855624) we should document that the approach in !877 (merged) is how we achieve this, by adding suitable words to tor-config's crate-level docs.

That will have a merge conflict with !878 (merged) and also really needs !877 (merged) to be merged, and !877 (merged) is blocked on !874 (merged). So I am filing this ticket for now.