src/sage/geometry/cone_critical_angles.py: add another "internal" war…

…ning

The existing warning about this module being internal is easy to
miss. This one should appear in bright red at the top of the module
documentation.

src/sage/geometry/cone_critical_angles.py

@@ -1,6 +1,13 @@
 r"""
 Find maximal angles between polyhedral convex cones
 
+.. WARNING::
+
+    This module is considered internal and its contents are subject to
+    change at any time without (deprecation) warning. The stable
+    interface is
+    :meth:`sage.geometry.cone.ConvexRationalPolyhedralCone.max_angle`.
+
 Finding the maximal (or equivalently, the minimal) angle between two
 polyhedral convex cones is a hard nonconvex optimization problem. The
 problem for a single cone was introduced in [IS2005]_, and was later