More documentation fixes.

Corrected a mistake in the explanation of the
C function Astronomy_GravSimInit: the `bodyStates`
parameter is NOT barycentric -- it is relative to the
originBody parameter.

Python had improperly formatted documentation for
Time.FromTerrestrialTime parameter `tt`.

The Python markdown generator `pydown` did not
correctly handle links to compound symbols like
`#GravitySimulator.Update`. It also was trying
to link to `StateVector[]` instead of `StateVector`.

Removed unnecessary and unhelpful documentation
for C# internal class constructors. They do not appear
in the generated markdown documentation anyway.

Other minor wording revisions in the documentation.

source/python/astronomy/astronomy.py

@@ -643,7 +643,8 @@ class Time:
 
         Parameters
         ----------
-        tt : The number of days after the J2000 epoch.
+        tt : float
+            The number of days after the J2000 epoch.
 
         Returns
         -------
@@ -9864,7 +9865,7 @@ class GravitySimulator:
     such as asteroids or comets, that move through the Solar System.
     It does so by calculating the gravitational forces on the bodies
     from the Sun and planets. The user of this class supplies a
-    list of initial positions and velocities for the bodies.
+    list of initial positions and velocities for the small bodies.
     Then the class can update the positions and velocities over small
     time steps.
     """