Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

apimachinery/pkg/runtime/doc: Split list into paragraphs

[Godoc uses go/doc][1] to convert comments to HTML, and go/doc [has no markup for lists][2].  With the previous comment, the lack of paragraph-splitting lines between the list entries was leading to entries 0, 1, etc. being formatted as a single paragraph.  And the hanging indents for entry 3, etc. were leading to formatting within `<pre>` blocks:

```console
$ curl -s https://godoc.org/k8s.io/apimachinery/pkg/runtime | grep -B1 -A7 'Your API objects'
<p>
0. Your API objects have a common metadata struct member, TypeMeta.
1. Your code refers to an internal set of API objects.
2. In a separate package, you have an external set of API objects.
3. The external set is considered to be versioned, and no breaking
</p>
<pre>changes are ever made to it (fields may be added but not changed
or removed).
</pre>
```

```release-note
NONE
```

[1]: https://godoc.org/golang.org/x/tools/cmd/godoc
[2]: https://golang.org/pkg/go/doc/#ToHTML