Add a note about mutually exclusive boolean flags.

When clap-rs/clap#2621 is resolved, we will be
able to derive a set of mutually exclusive boolean flags from an enum
in a struct that derives clap::Parser.

builtin_apps/src/get.rs

@@ -54,6 +54,8 @@ pub fn run<'list>(list: &'list TodoList, cmd: &Get) -> PrintableResult<'list> {
             // By default, show all context.
             (false, false, false) => (true, true),
             // Any other combination is invalid.
+            // TODO: we will be able to avoid this unreachable!() once
+            // https://github.com/clap-rs/clap/issues/2621 is resolved.
             _ => unreachable!(),
         };
     let tasks_to_print = requested_tasks