[Doc request] Multi-value tags in metadata

[andrewbaxter]

In some media tags can have multiple values. Looking at the documentation of tags() and Tag there's no mention of this, and tag.value() is a single string not an array.

In the code it's somewhat documented (if a user wants to dig that far): https://github.com/pdeljanov/Symphonia/blob/master/symphonia-metadata/src/id3v2/frames.rs#L647

I have one other question, which is whether the tag value order is preserved. I see the tag is push ed so I assume it's an insertion-order container, but I'm not sure if additional processing happens afterward.