Fix #8756: doc: `extra-source-files` are tracked by `cabal build`

Changing a file from `extra-source-files` causes `cabal build` to
reevaluate the build plan, judging from the following behavior.

```console
$ cabal build
...

$ cabal build
Up to date

  # Make a change to any of the extra-source-files, e.g. Foo.txt

$ cabal build
...
In order, the following would be built (use -v for more details):
 - ... (lib) (file Foo.txt changed)
...
```

One notes here that the 3rd `cabal build` does not just output "Up to date".