rustdoc: improve attribute span handling in diagnostics

In #150721 intra-doc links are rendered for deprecation notes.

Currently the span that is used when reporting an error (e.g. the link cannot be resolved) is a bit too broad. A span containing all doc comments and the deprecated attribute is used as a fallback.

Error reporting would improve slightly if just the span of the deprecation note would be used. That requires that that span is stored, and then used in report_diagnostic.