Refine text around extras for non-packages

Per suggestion and with additional clarification.

peps/pep-0735.rst

@@ -97,8 +97,11 @@ package specifiers which are published as part of a package's metadata, and
 which a user can request under that name, as in ``pip install 'foo[bar]'`` to
 install ``foo`` with the ``bar`` extra.
 
-Because ``extras`` are package metadata, they are not necessarily usable when
-a project does not build a distribution (i.e., is not a package).
+Because ``extras`` are package metadata, they are not guaranteed to be
+statically defined and may require a build system to resolve.
+Furthermore, definition of a ``[project.optional-dependencies]`` indicates to
+many tools that a project is a package, and may drive tool behaviors such as
+validation of the ``[project]`` table.
 
 For projects which are packages, ``extras`` are a common solution for defining
 development dependencies, but even under these circumstances they have