docs: generateMetadata values should be serializable with use cache (#97551)
### Why?
generateMetadata can be marked with use cache, but Cache Function return
values must be serializable. The metadataBase examples use URL objects,
so the interaction needs co-located guidance for cached metadata.
### How?
Add a Good to know note that recommends returning metadataBase as a
string, such as with url.toString(), and links to the use cache
serialization requirements. Runtime behavior is unchanged.
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Co-authored-by: Joseph <joseph.chamochumbi@vercel.com>