Don't lint unnamed consts and nested items within functions in `missing_docs_in_private_items`

[y21]

With this change we no longer require doc comments for const _: () items as well as nested items in functions or other bodies. In both of those cases, rustdoc generates no documentation even with --document-private-items.

Fixes #13427 (first commit) Fixes #13298 (second commit) cc https://github.com/rust-lang/rust-clippy/issues/5736#issuecomment-668524296

changelog: [missing_docs_in_private_items]: avoid linting in more cases where rustdoc generates no documentation