Link reference in `dyn` keyword documentation
The "read more" sentence formatted "object safety" as inline code
instead of providing a link to more information. This PR adds a link
to the Reference about this matter, as well as the page regarding trait
objects.
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We could also put these links in the very first line (instead of the link to the
Book) and in the first paragraph which mentions the "object safe" requirement.
Personally, I think it's good to keep the link to the Book up-front as it's more
accessible than the Reference.
0d14acad7e