Document and complement XInclude failures as warnings on translations

[alfsb]

This PR better document how XInclude failures are treated on translations. It only cares for <xi:include> without <xi:failback>, so it leaves open <xi:failback> usage for other situations.

In fact, I tested this on all languages with doc-en stripped of <xi:failback>.

As an recommendation, I suggest mass removing all mentions of <xi:failback> on doc-en, down to XML comments, after 8.4 doc changes calms down, and possibly further creating an github action/trigger that recuses usage of <xi:failback>, linking this trigger to mention XInclude failures are soft errors on translations. on configure.php. The reason being that failed XInclude leaves different artifacts in the presence and absence of <xi:failback>. Caring for both is very confusing.

[alfsb]

I would like comments to making code comments and messages into idiomatic English (not my first language).

Also, I was thinking in placing some default message in the patched up nodes, something like %xinclude fail%, that is something small but visible, to indicate patched up nodes into rendered manual. The actual implementation creates empty tags, that may be more confusing that an small error message.

[Girgias]

MSTM.

[alfsb]

Changed the texts a little, I plan to merge this tomorrow.

[alfsb]

Merged. I expect no breakages, but let me know if are any problems. This enable removing empty <xi:fallback> on all languages, as they are now, and allows for <xi:fallback> be used for other cases.