DOC: say where a Monte Carlo run's seed goes

simulate() gained random_seed with no entry in its own Parameters
block, and the stochastic page hands users to the MonteCarlo class
without saying that a run is fixed there rather than on the models.

Signed-off-by: thc1006 <84045975+thc1006@users.noreply.github.com>

Author: thc1006

docs/user/stochastic.rst

@@ -289,3 +289,11 @@ better reflecting the inherent uncertainties in rocketry.
 .. note::
     See the ``MonteCarlo`` class documentation for more information on how to run \
     Monte Carlo simulations with stochastic objects.
+
+.. note::
+    A whole run is fixed by ``MonteCarlo.simulate(random_seed=...)`` rather than
+    by seeding these models yourself. Each simulation takes its seed from its
+    own index, so simulation 7 draws the same inputs whether the run was serial
+    or split over any number of workers, and appending with the same seed
+    carries the same stream on. Without it a run draws fresh entropy and
+    reproduces nothing.

rocketpy/simulation/monte_carlo.py

@@ -339,6 +339,20 @@ def simulate(
             number of workers will be equal to the number of CPUs available.
             A minimum of 2 workers is required for parallel mode.
             Default is None.
+        random_seed : int, sequence of int or numpy.random.SeedSequence, optional
+            Fixes what every simulation draws. Simulation ``i`` takes the same
+            inputs whichever way the run was split up, so serial and parallel
+            results agree and the number of workers does not reach the
+            sampling. Keyword-only. Default is None, which draws fresh entropy
+            and reproduces nothing.
+
+            Appending continues the same stream when the same seed is given
+            again, since an index maps to a seed and to nothing else. Nothing
+            here records the seed, so nothing here can tell you that a later
+            append was given the same one; that is #1075.
+
+            A ``Generator`` or ``BitGenerator`` is refused rather than read.
+            Pass the seed it was built from.
         kwargs : dict
             Custom arguments for simulation export of the ``inputs`` file. Options
             are: