Apply Rails/InverseOf rubocop rule

Not doing so has a few gotchas when working with relations, particularly
with records which are not stored in the database.

I'm excluding the related content file because it's got a very peculiar
relationship with itself: the `has_one :opposite_related_content` has no
inverse; the relation itself is its inverse. It's a false positive since
the inverse condition is true:

```
content.opposite_related_content.opposite_related_content.object_id ==
  content.object_id
```

app/models/concerns/documentable.rb

@@ -2,7 +2,7 @@ module Documentable
   extend ActiveSupport::Concern
 
   included do
-    has_many :documents, as: :documentable, dependent: :destroy
+    has_many :documents, as: :documentable, inverse_of: :documentable, dependent: :destroy
     accepts_nested_attributes_for :documents, allow_destroy: true
   end