Minor doc modif in XmlDocument

XmlDocument::parse (const File& file) and XmlDocument::parse (const String& xmlData)
now return std::unique_ptr<XmlElement>, so the following is outdated and could be removed from their doc :

an even better shortcut is the juce::parseXML() function, which returns a std::unique_ptr!

I think it’s still recommended to use juce::parseXML() over juce::XmlDocument::parse, so the comment should just remove the part about returning a unique_ptr.

Interestingly, the implementations of those functions are identical. I suppose XmlDocument::parse has been kept around backwards compatibility since it was around before the free functions were added?