Better document the behaviour of Inspire_Partisans

Including new changes.

docs/Modding/Rulesets/effects.rst

@@ -809,6 +809,14 @@ Has_Senate
 
 Inspire_Partisans
     Partisan units (defined in :file:`units.ruleset`) may spring up when this player's cities are taken.
+    The number of partisans created is a function of the city size.
+    When multiple unit types have the Partisan flag, the game tries to choose one the player can build. If
+    the player cannot build any, the first Partisan unit type is used. If the created partisans units have a
+    non-zero ``pop_cost``, the city will lose citizens accordingly.
+
+    Note that this effect can create units that the player could normally not build (because the tech hasn't
+    been researched or the ``NoBuild`` flag is set. It is your responsibility to set the appropriate
+    requirements to avoid creating advanced units if a city is conquered in the early game.
 
 .. _effect-happiness-to-gold: