bootstrap.sh now also builds documentation of installed packages

Regularly, when I install a new version of GHC and cabal-install, I modify the bootstrap.sh file to also build the documentation of installled packages.

I do this because otherwise `ghc-pkg check` reports missing documentation for the respective packages, and I like `ghc-pkg check` to output no issues to be sure of a clean, working installation.

You may ignore this pull request if you wish, as this is not an killer feature, but I wanted to share my idea, finally.