Fix discover field detail messages not updating when documents update

When updating filters/queries/etc. the message inside a discover field details may need to change (see #2249), but prior to this change it wasn't updating. This is because we were using `bo-if` (bind once) instead of `ng-if`. This PR just changes them to `ng-if`. The only potential worry is that this could negatively affect performance in examples where there are many, many fields, but we can deal with that situation if it causes problems.

Closes #2249.