Document that a None checkpoint can mean an unreachable server

``omni.client`` reports a checkpoint that was never published and a
server it cannot reach identically, so both leave the download raising
FileNotFoundError and both return None. The printed message says so, but
the docstring and the changelog described None as an unpublished
checkpoint only, inviting callers to read a transient outage as a
confirmed absence.

Author: AntoineRichard

source/isaaclab_rl/changelog.d/antoiner-pretrained-checkpoint-errors.minor.rst

@@ -13,5 +13,5 @@ Changed
   ``RuntimeError`` when a published checkpoint cannot be downloaded, for instance when the
   ``.pretrained_checkpoints`` cache directory is not writable, instead of returning ``None``. The
   originating error is chained as the cause. ``None`` is now returned only when the asset server does not
-  provide the checkpoint, so callers that treat ``None`` as "no checkpoint published for this task" are
-  unchanged; callers that relied on ``None`` to mask local download failures must catch ``RuntimeError``.
+  report the checkpoint, which covers both an unpublished checkpoint and a server that could not be
+  reached; callers that relied on ``None`` to mask local download failures must catch ``RuntimeError``.

source/isaaclab_rl/isaaclab_rl/utils/pretrained_checkpoint.py

@@ -239,8 +239,11 @@ def get_published_pretrained_checkpoint(
             to use the legacy checkpoint layout.
 
     Returns:
-        The path, or None when the asset server does not publish a checkpoint for this
-        task and backend combination. The reason is printed before returning.
+        The path, or None when the asset server does not report a checkpoint for this task
+        and backend combination. That covers both a checkpoint that was never published and
+        a server that could not be reached, which ``omni.client`` does not distinguish, so a
+        transient outage is not evidence that a checkpoint does not exist. The reason is
+        printed before returning.
 
     Raises:
         RuntimeError: If the checkpoint is published but could not be downloaded, for