Document the 15625 scene partition cap for Isaac RTX renderer (#7573)

## Summary
- Document that Isaac RTX's `rtx.scenedb.plugin` caps the number of
scene partitions at 15625; requesting more environments than that with
scene partitioning enabled silently discards additional partitions and
causes environments to share tiled camera views.
- Add a warning note to the scene partitioning section of the renderers
overview doc.
- Add a corresponding Known Issues entry under the Renderers section,
following the existing animated-curve scene-partition entry's format.

## Type of change

- Documentation update

## Release backport

- [x] <!-- backport-active-release --> Backport this pull request to the
active release branch after it merges into `develop`

## Test plan
- [x] `uv run isaaclab -f` passes (docs/rst formatting checks)
- [x] Manually reviewed rendered structure/headings match existing doc
conventions in both files

(cherry picked from commit 5c78cae)

Author: mataylor-nvidia

docs/source/overview/core-concepts/renderers.rst

@@ -88,6 +88,16 @@ token per instance; markers without that ownership information remain shared.
    culled once they deform beyond their initial extent. See
    :ref:`known-issues-animated-curve-scene-partition` for the workaround.
 
+.. warning::
+
+   The Isaac RTX and OVRTX renderers cap the number of scene partitions at 15625.
+   Requesting more than 15625 environments with scene partitioning enabled discards
+   the additional partitions, and ``rtx.scenedb.plugin`` logs ``SceneDbContext : Maximum
+   number of scene partitions (15625) reached. Additional scene partitions will be
+   discarded.`` Environments beyond that count then share a partition with another
+   environment, so their tiled camera views can show another environment's geometry.
+   See :ref:`known-issues-scene-partition-count-cap` for details.
+
 Architecture Overview
 ---------------------
 

docs/source/refs/issues.rst

@@ -196,6 +196,30 @@ There are two workarounds:
   ``False`` to opt out of partitioning entirely, at the cost of the per-environment
   culling.
 
+.. _known-issues-scene-partition-count-cap:
+
+Scene partitioning is capped at 15625 partitions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+**Affects:** ``renderer=isaacsim_rtx`` with scene partitioning enabled, and ``renderer=ovrtx``.
+
+The underlying ``rtx.scenedb.plugin`` allocates a fixed-size pool of scene partitions and
+caps it at 15625, regardless of which renderer requests them. Isaac Lab assigns one scene
+partition per environment when
+:attr:`~isaaclab_physx.renderers.IsaacRtxRendererCfg.enable_scene_partitioning` is enabled
+for the Isaac RTX renderer, and OVRTX always assigns one scene partition per environment, so
+runs with more than 15625 environments exceed the pool on either backend. Once the cap is
+hit, ``rtx.scenedb.plugin`` logs a warning and discards the remaining partitions:
+
+.. code-block:: text
+
+    [Warning] [rtx.scenedb.plugin] SceneDbContext : Maximum number of scene partitions
+    (15625) reached. Additional scene partitions will be discarded.
+
+Environments beyond the cap are left without their own partition and end up sharing one
+with another environment, so their tiled camera views can render another environment's
+geometry instead of their own.
+
 Using instanceable assets for markers
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~