For example, from inside the current stack project:

• stack docs Cabal opens
  ~/.stack/snapshots/x86_64-linux/lts-3.14/7.10.2/doc/Cabal-1.22.4.0/index.html
• stack --stack-yaml stack-7.8.yaml docs Cabal opens
  ~/.stack/snapshots/x86_64-linux/lts-2.22/7.8.4/doc/Cabal-1.18.1.5/index.html

Given that stack already installs the documentation for ghc and haddock together with the binaries, stack docs could also open these. There needs to be a way to distinguish between the library documentation for haddock and the user guide though.