Usually, binary distributions include Haddock documentation pages for all the bundled libraries with source links (the sources have historically been generated with HsColour, although with Hadrian binary dists we've moved to using Haddock's own Hyperlinker backend). It looks like 8.6.4 bindists are missing the source links and files altogether:

$ l ~/.ghcup/ghc/8.6.4/share/doc/ghc-8.6.4/html/libraries/base-4.12.0.0/src
ls: /Users/atheriault/.ghcup/ghc/8.6.4/share/doc/ghc-8.6.4/html/libraries/base-4.12.0.0/src: No such file or directory
$ l ~/.ghcup/ghc/8.6.3/share/doc/ghc-8.6.3/html/libraries/base-4.12.0.0/src
total 21400
drwxr-xr-x  240 atheriault  staff   8.0K Dec 14 08:30 .
drwxr-xr-x  231 atheriault  staff   7.7K Dec 14 08:30 ..
-rw-r--r--    1 atheriault  staff    26K Dec 14 08:30 Control-Applicative.html
-rw-r--r--    1 atheriault  staff    68K Dec 14 08:30 Control-Arrow.html
-rw-r--r--    1 atheriault  staff    11K Dec 14 08:30 Control-Category.html
-rw-r--r--    1 atheriault  staff    19K Dec 14 08:30 Control-Concurrent-Chan.html
...