docs: clarify packing of UUID in ffa manifest

Update the ffa-manifest-bindings to clarify how the
Partition property 'UUID' must be packed.

Signed-off-by: Sami Mujawar <sami.mujawar@arm.com>
Change-Id: Ic5ee67c9606dec30ed3a3234a5f40a976ecbb72c
diff --git a/docs/components/ffa-manifest-binding.rst b/docs/components/ffa-manifest-binding.rst
index 2b6382b..6fca245 100644
--- a/docs/components/ffa-manifest-binding.rst
+++ b/docs/components/ffa-manifest-binding.rst
@@ -31,6 +31,10 @@
    - value type: <prop-encoded-array>
    - An array consisting of 4 <u32> values, identifying the UUID of the service
      implemented by this partition. The UUID format is described in RFC 4122.
+   - These 4 <u32> values are packed similar to the UUID register mapping
+     specified in section '5.3 Unique Identification format', SMC Calling
+     Convention, DEN0028, v1.6 G BET0
+     (https://developer.arm.com/documentation/den0028/latest/).
 
 - id
    - value type: <u32>