Document full component qualification syntax in docs

The docs mention `lib:` and `exe:` in passing but don't give an exhaustive list of all prefixes, which are set here:

https://github.com/haskell/cabal/blob/00a2351789a460700a2567eb5ecc42cca0af913f/Cabal/src/Distribution/Simple/BuildTarget.hs#L569

Further, they don't describe constraint scopes, i.e. that the `any` qualifier can be used in cabal.project to describe the _level_ of a dependency, in terms of if it is a top level dependency or a subdependency due to setup or build-tools depends, etc.

This is set here:

https://github.com/haskell/cabal/blob/00a2351789a460700a2567eb5ecc42cca0af913f/cabal-install/src/Distribution/Client/Targets.hs#L728

This stuff should be enumerated more carefully in the docs somewhere.