Expansion of content files causes bogus warnings such as

   :0: warning: Can't find tag end: ]> in docs for: module-tutorial-tool.xml file.

where module-tutorial-tool.xml is the name of the expanded content file.  This seems to be a regression.

Why it happens?

Function ExpandAbbreviationsEndTag() asserts that <!DOCTYPE declaration ends with ]>, not just >.  But the square brackets, that enclose internal subset declaration if present, are optional.  Content files rarely need to declare internal subsets so they generally do not contain any square brackets in the doctype declaration (heck, the brackets are not even present in files generated by gtk-doc itself). Requiring them makes no sense.