[css-ruby] Rule of line breaking between ruby bases

The current spec says:

> Whether ruby can break between two adjacent ruby bases is controlled by
normal line-breaking rules for the base text, exactly as if the ruby bases
were adjacent inline boxes.

I propose that we should add that, there is always a soft wrap opportunity
between ruby bases.

There are two reasons:
1. For real world use cases, we nearly always use ruby with CJK characters
as bases. Even if we use latins in bases, we generally won't spread one
word into several bases. Hence announcing this doesn't affect real world
use cases.
2. All impls including WebKit, Trident, and current Gecko behave in this
way.

I think there is no reason for the spec to cover an imaginary use case
against UA impls.

- Xidorn