Clarification on omegaconf's interpolation support in LightningCLI (#12571)

* Clarification on omegaconf's interpolation support in LightningCLI.

* Update docs/source/common/lightning_cli.rst

Co-authored-by: Adrian Wälchli <aedu.waelchli@gmail.com>
Co-authored-by: Akihiro Nitta <nitta@akihironitta.com>

docs/source/common/lightning_cli.rst

@@ -813,6 +813,12 @@ With the encoder-decoder example model above a possible YAML that uses variable
       - ${model.encoder_layers}
       - 4
 
+.. note::
+
+    OmegaConf's interpolation only works within a single config file. Trying to interpolate across command line
+    arguments or using it in `parser.set_defaults` will not work. More up to date information about the interpolation
+    support can be found `here <https://jsonargparse.readthedocs.io/en/stable/#variable-interpolation>__`.
+
 
 Optimizers and learning rate schedulers
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^