Add comments in the Kerr doctests

src/sage/manifolds/catalog.py

@@ -127,6 +127,9 @@ def Kerr(m=1, a=0, coordinates="BL", names=None):
         4-dimensional Lorentzian manifold M
         sage: K.atlas()
         [Chart (M, (t, r, th, ph))]
+
+    The Kerr metric in Boyer-Lindquist coordinates (cf. :wikipedia:`Kerr_metric`)::
+
         sage: K.metric().display()
         g = (2*m*r/(a^2*cos(th)^2 + r^2) - 1) dt⊗dt
          - 2*a*m*r*sin(th)^2/(a^2*cos(th)^2 + r^2) dt⊗dph
@@ -135,6 +138,8 @@ def Kerr(m=1, a=0, coordinates="BL", names=None):
          - 2*a*m*r*sin(th)^2/(a^2*cos(th)^2 + r^2) dph⊗dt
          + (2*a^2*m*r*sin(th)^2/(a^2*cos(th)^2 + r^2) + a^2 + r^2)*sin(th)^2 dph⊗dph
 
+    The Schwarzschild spacetime with the mass parameter set to 1::
+
         sage: K.<t, r, th, ph> = manifolds.Kerr()
         sage: K
         4-dimensional Lorentzian manifold M
@@ -144,6 +149,9 @@ def Kerr(m=1, a=0, coordinates="BL", names=None):
         sage: K.default_chart().coord_range()
         t: (-oo, +oo); r: (0, +oo); th: (0, pi); ph: [-pi, pi] (periodic)
 
+
+    The Kerr spacetime in Kerr coordinates::
+
         sage: m, a = var('m, a')
         sage: K.<t, r, th, ph> = manifolds.Kerr(m, a, coordinates="Kerr")
         sage: K