docs: Link pyproject.toml to ext_modules

Author: abitrolly

docs/userguide/pyproject_config.rst

@@ -96,9 +96,8 @@ file, and can be set via the ``tool.setuptools`` table:
 Key                       Value Type (TOML)           Notes
 ========================= =========================== =========================
 ``py-modules``            array                       See tip below.
-``ext-modules``           array of                    **Experimental** - Each item corresponds to a
-                          tables/inline-tables        :class:`setuptools.Extension` object and may define
-                                                      the associated parameters in :wiki:`kebab-case`.
+``ext-modules``           array of                    **Experimental**.
+                          tables/inline-tables        See :doc:`/userguide/ext_modules`.
 ``packages``              array or ``find`` directive See tip below.
 ``package-dir``           table/inline-table          Used when explicitly/manually listing ``packages``.
 ------------------------- --------------------------- -------------------------