Merge branch 'mauro' into docs-next

Mauro sez:

Patches 1 to 5 contain changes to the documentation toolset:

- The first 3 patches help to reduce a lot the number of reported
  kernel-doc issues, by making the tool more smart.

- Patches 4 and 5 are meant to partially address the PDF
  build, with now requires Sphinx version 2.4 or upper.

The remaining patches fix broken references detected by
this tool:

        ./scripts/documentation-file-ref-check

and address other random errors due to tags being mis-interpreted
or mis-used.

They are independent each other, but some may depend on
the kernel-doc improvements.