[docs] lot of different inputs require proper xml escaping

[SaculRennorb]

I initially encountered this issue while using buyer information that contained a \& in the name, which produces invalid xml if left unescaped.

I don't expect the packege to do the escaping for me, seeing as this might be quite expensive to always do on all fields, but some remarks should be added into the documentation about this imo.

[TeXhackse]

Thanks for the suggestion. It's actually not that expensive as the mechanism is already there and could be extended. But I will think about it. Generally I'd not do replacements like this by default, as that's a task a personal wrapper package which depends on your input should resolve.