Arm backend: Update backends-arm-ethos-u.md. (pytorch#14255)

Update the doc to be more focused on detailed API documentation.

Note that this documents the change introduced in pytorch#14191.

---------

Signed-off-by: Adrian Lundell <[email protected]>

Author: AdrianLundell

backends/arm/ethosu/partitioner.py

@@ -15,6 +15,14 @@
 
 @final
 class EthosUPartitioner(TOSAPartitioner):
+    """
+    Partitions subgraphs supported by the Arm Ethos-U backend.
+
+    Attributes:
+        compile_spec: List of CompileSpec objects for Ethos-U backend.
+        additional_checks: Optional sequence of additional operator support checks.
+    """
+
     def __init__(
         self,
         compile_spec: EthosUCompileSpec,