Quickbook warning

From build output:
```
Generating output file: ../../bin.v2/libs/json/doc/json_doc.xml
/home/grisumbras/dev/cppal/boost/bin.v2/libs/json/doc/reference.qbk|92| warning: line breaks generate invalid boostbook (will only note first occurrence).
```

reference.qbk:92
```
[Constructor. [br][role silver —][br]Copy constructor. [br][role silver —][br]Pilfer constructor. [br][role silver —][br]Move constructor.
```

I've looked through quickbook sources, the tool seems to warn on _any_ use of `[br]`, so the only way to remove the warning is probably to use paragraphs.