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 }
```

0 files changed, 0 insertions, 0 deletions