src/doc/en/reference/discrete_geometry/index.rst: add cone critical a…

…ngles

This is an internal module, but it's fully documented and nice to have
access to the implementation behind the max_angle() function. Despite
being documented, the functions in cone_critical_angles.py are subject
to change at any time. This itself is loudly documented.

src/doc/en/reference/discrete_geometry/index.rst

@@ -80,6 +80,7 @@ Toric geometry
    sage/geometry/toric_lattice
    sage/geometry/cone
    sage/geometry/cone_catalog
+   sage/geometry/cone_critical_angles
    sage/geometry/fan
    sage/geometry/fan_morphism
    sage/geometry/point_collection