Constrain certain descendant content in ruby container.

[skynavga]

Proscribe the following from appearing in a ruby container:

• use of span children of container that do not specify a ruby style, i.e., that do not play a role in ruby structure; if present, then such span is ignored for presentation purposes;
• use of #PCDATA children of container, baseContainer, and textContainer; i.e., permit #PCDATAA only in base, text, and delimiter; if used, then ignore for presentaion purposes;
• use of br element in ruby or its descendants, and, if used, treat as normal (collapsible) whitespace;
• use of U+2028 (LINE SEPARATOR) and U+2029 (PARAGRAPH SEPARATOR) in ruby or its descendants, and, if used, they are interpreted as normal (collapsible) whitespace.

Editorial notes from 2015-02-18.

[cconcolato]

IMO, the text would be much more readable if it was expressed as it is currently in the note, i.e.:

container
: base text
| base delimiter text delimiter
| baseContainer textContainer textContainer?

rather than with a lengty bullet point list.

[skynavga]

We can't express the bulleted constraints using this grammar. I am working on this issue now, so a PR will appear shortly.