Add test/doc for :if/:except in skip_before_action

The new test/docs further explain the conflicts that can happen when
mixing `:if`/`:unless` options with `:only`/`:except` options in
`skip_before_action`.

The gist is that "positive" filters always have priority over negative
ones.

The previous commit already showed that `:only` has priority over `:if`.

This commit shows that `:if` has priority over `:except`.

For instance, the following snippets are equivalent:

```ruby
skip_before_action :some_callback, if: -> { condition }, except: action
```

```ruby
skip_before_action :some_callback, if: -> { condition }
```

1 files changed, 5 insertions, 0 deletions

diff --git a/actionpack/lib/abstract_controller/callbacks.rb b/actionpack/lib/abstract_controller/callbacks.rb
index 865bd6bbe0..8571383739 100644
--- a/actionpack/lib/abstract_controller/callbacks.rb
+++ b/actionpack/lib/abstract_controller/callbacks.rb
@@ -33,6 +33,11 @@ module AbstractController
       #
       #   only: :index, if: -> { true } # the :if option will be ignored.
       #
+      # Note that <tt>:if</tt> has priority over <tt>:except</tt> in case they
+      # are used together.
+      #
+      #   except: :index, if: -> { true } # the :except option will be ignored.
+      #
       # ==== Options
       # * <tt>only</tt>   - The callback should be run only for this action
       # * <tt>except</tt>  - The callback should be run for all actions except this action