syntax: add a some docs/clarification to the fields of ExpnInfo.
syntax: add a missing span rewrite in fold.

This was leaving Decls without the new spans; this is a minor change,
since literally nothing reads in the code base reads the span of a Decl
itself, always just its contents.
auto merge of #13152 : huonw/rust/wtf-are-things-in-spans, r=alexcrichton

Add some docs to ExpnInfo. Add a single overlooked `new_span` call to the folder (I'm pretty sure nothing reads this span, though, so it's probably pointless).