Large Haddock submodule merge landing soon

Hi all, I have a very large Haddock change (merging Haddock's development branch, `ghc-8.10`, into the `ghc-head` branch) pending in !4819. I will try merge it as soon as Marge's next batch finishes. If you have a Haddock change outstanding you will need to perform a rather significant rebase after this change goes in. However, I am available to help with rebase if needed; just send me the MR number of your merge request and I can take of the rest. Cheers, - Ben