Fix #8756: doc: extra-source-files are tracked by cabal build (#8765

)

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".

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

doc/cabal-package.rst

@@ -774,9 +774,9 @@ describe the package as a whole:
 
 .. pkg-field:: extra-source-files: filename list
 
-    A list of additional files to be included in source distributions
-    built with :ref:`setup-sdist`. As with :pkg-field:`data-files` it can use
-    a limited form of ``*`` wildcards in file names.
+    A list of additional files to be included in source distributions built with :ref:`setup-sdist`.
+    As with :pkg-field:`data-files` it can use a limited form of ``*`` wildcards in file names.
+    Files listed here are tracked by ``cabal build``; changes in these files cause (partial) rebuilds.
 
 .. pkg-field:: extra-doc-files: filename list
     :since: 1.18