Documentation fix.
Small warning.
When unpacking a tarball for install, rename 'dist' to useDistPref.

Allows installing packages that include pre-generated files (e.g. alex, happy)
into a sandbox even when the required preprocessor is not installed on the system.
Merge pull request #1578 from 23Skidoo/happy-alex-sandbox

When unpacking a tarball for install, rename 'dist' to useDistPref.
Sandbox config: ignore the value of 'symlink-bindir' from '~/.cabal/config'.

Fixes #1514.