Clarify "can contain" documentation and error handling #2702
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All "can contain" entries have been split into "can contain at most one"
and "can contain any number". Additionally, there is now a note to
implementors at the top of the specification listing how to handle parsing
errors. (More of a reminder on how to handle XML errors in general, since
TMX is a form of XML.)
Closes #696, #1754