Unifying the various documentation flags under the `documentation:` stanza

(_Please read https://github.com/haskell/haskell-language-server/issues/4210 first_)

There seems to be a fragmentation of the various ways to generate Haddocks on a cabal project, and this confuses end-users.

The most ergonomic option, setting `documentation: True` in one's cabal.project file, is actually not going all the way through, which is quite surprising:

> documentation: true does not imply haddock-all, haddock-benchmarks, haddock-executables, haddock-internal or haddock-tests. These need to be enabled separately if desired.

– https://cabal.readthedocs.io/en/stable/cabal-project-description-file.html#cfg-field-documentation

And according to the ticket linked above, it doesn't pass the `-haddock` option to GHC, which is what HLS needs for proper displaying of the documentation through LSP.

1. Is there a reason about the aforementioned caveat of `documentation:`?
2. I think we should probably pass `-haddock` to GHC when `documentation:` is set to True. Any issue one can see with this?

Read the linked HLS issue #4210 first, then review the Cabal project description documentation for the `documentation:` stanza and its related Haddock flags. The issue does not name repository files or tests; the work is done only after the intended flag behavior and GHC integration are agreed and verified.

Written by the indexing model from the issue text.