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 #5736 (comment)

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

[rustbot]

r? @Centri3

rustbot has assigned @Centri3.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[Centri3]

Thanks, @bors r+

[bors]

📌 Commit b3bf128 has been approved by Centri3

It is now in the queue for this repository.

[bors]

⌛ Testing commit b3bf128 with merge ddd1a86...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: Centri3
Pushing ddd1a86 to master...