Add Constraint AASd-133 in SpecificAssetId documentation (#147)

This adds a documentation of AASd-133 to `SpecificAssetId`.

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Co-authored-by: s-heppner <s.heppner@plt.rwth-aachen.de>

Author: zrgt

basyx/aas/model/base.py

@@ -2130,6 +2130,9 @@ class SpecificAssetId(HasSemantics):
     A specific asset ID describes a generic supplementary identifying attribute of the asset.
     The specific asset ID is not necessarily globally unique.
 
+    *Constraint AASd-133:* SpecificAssetId/externalSubjectId shall be a global reference,
+        i.e. Reference/type = ExternalReference
+
     :ivar name: Key of the identifier
     :ivar value: The value of the identifier with the corresponding key.
     :ivar external_subject_id: The (external) subject the key belongs to or has meaning to.