docs: moving-circle docstrings — tables-attached guard, vx/vy meaning, center keyword

Docstring and documentation wording only; no code change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/moving_bodies.md

@@ -14,7 +14,7 @@ cells (M1 `MovingCircleCells`), moving-wall Bouzidi with wall velocity
 `u + omega x r`, and separate per-body diagnostics.
 
 *API.*
-- `Simulation.add_body("circle", centre, radius,
+- `Simulation.add_body("circle", center=(x, y), radius=r,
   translation=Translation(offset_box, max_speed))` before `initialize()`;
   `offset_box = (x_min, x_max, y_min, y_max)` in lattice cells around the
   base centre (must contain 0), `0 < max_speed ≤ .5`.

src/CelerisLab/simulation.py

@@ -122,8 +122,9 @@ class Simulation:
                 D2Q9/FP32/unshifted/DB only; set before initialization or stage
                 a new body. No runtime motion or amplitude setter.
             translation: ``body.motion.Translation`` declaring a translating
-                circle (before initialize only; docs/moving_bodies.md). Until
-                the moving kernels exist, positive-step runs are rejected.
+                circle (before initialize only; docs/moving_bodies.md).
+                Positive-step runs are rejected until ``initialize()`` has
+                attached its moving tables.
             **kwargs: passed to the geometry constructor.
 
         Returns:
@@ -360,8 +361,11 @@ class Simulation:
         Args:
             id: body_id from add_body().
             omega: rotation speed (lattice units).  ``None`` = no change.
-            vx: translation velocity x (future).  ``None`` = no change.
-            vy: translation velocity y (future).  ``None`` = no change.
+            vx: translation velocity x. For a translating circle: the
+                interval wall velocity of the target, only together with
+                ``offset``; otherwise stored in the body state (not applied
+                to stationary walls).  ``None`` = no change.
+            vy: translation velocity y; same rules as ``vx``.
             offset: target centre offset of a translating circle; with vx/vy
                 (default 0) it replaces the pending target after validation
                 against the applied offset (docs/moving_bodies.md, 4a). Only