Auto merge of #111512 - petrochenkov:microdoc2, r=GuillaumeGomez

rustdoc: Cleanup doc string collapsing

`doc_value` and (former) `collapsed_doc_value` can be implemented in terms of each other, and `doc_value` doesn't need the `Option`.

This PR doesn't do any semantic changes, only refactoring, although some pre-existing choices between `doc_value` and `collapsed_doc_value` across rustdoc may be questionable.

Author: bors