Haddock no longer writes compilation files by default, so we do not need to pass
tmp dirs for `-hidir`, `-stubdir`, and `-odir` via `--optghc`. Indeed, we do not
*want* to do so, since it results in recompilation for every invocation of
Haddock via Cabal. This commit stops this from happening for haddock versions >=
2.28 (when Hi Haddock was introduced).

This commit also stops the default definition of the `__HADDOCK_VERSION__` macro
when invoking GHC through haddock. Since a very limited set of users may still
depend on this macro, we introduce the `--haddock-version-cpp` flag and
`haddock-version-cpp:` cabal.project field, which enable the definition of
the `__HADDOCK_VERSION__` macro when invoking GHC through Haddock. This will
almost guarantee recompilation during documentation generation due to the macro
definition.

This commit also renames the `--haddock-lib` flag to `--haddock-resources-dir`
(and `haddock-lib:` cabal.project field to `haddock-resources-dir:`), and adds
this flag to the users guide since it was missing an entry. This also allows us
to add this field to `cabal-install:test:integration-tests2`, since it is no
longer ambiguous with the `--lib` flag.

This commit also causes `documentation: true` or `--enable-documentation` to
imply `-haddock` for GHC.

Also, since Haddock >= 2.29 is renaming `--lib` to `--resources-dir`, this
commit switches the flag provided to Haddock using a backwards compatible
condition based on the Haddock version.

Adds a changelog entry.