Add note about xml:id valid values, and improve examples

[nigelmegitt]

Feedback from @magicbadger:

the contents of the xml:id attribute don’t jump out at me.

Also:

The thing that perhaps caught me is not appreciating that I was creating “Script Events” and that in that case, “xml:id” is mandatory. The error likely comes from me seeing some fairly lean examples at the start of the document that do not have an xml:id. Perhaps it could be worth giving some different use case examples towards the start of the document?

Suggested actions:

• add note that each xml:id attribute must have a unique value within the document and a summary of the formatting requirements, e.g. must not begin with a digit.
• make sure the introduction examples include xml:id on <div> elements