lint mode for missing docs

[metajack]

I would like to see a lint mode that could warn on missing documentations, hopefully by type. Something like -Wdoc-trait or -Wdoc-struct to warn on missing docs on traits or structs, etc.

[graydon]

nominating for well-covered.

[graydon]

accepted for well-covered milestone

[alexcrichton]

After #6715 and #6798, I think this can be closed. If there are missing things that need lints for documentation, I think they should have individual bugs.