Document `slot_uri` override rationale in schema annotations

[StroemPhi]

Affected: chemical_entities_ap.yaml, chemical_reaction_ap.yaml, material_entities_ap.yaml
Severity: Documentation
Status: Not started

Problem: ChemDCAT-AP's YAML schema files use SIO:000008 instead of DCAT-AP+'s default dcterms:relation on attribute sub-slots, but no notes or comments annotation in the schema explains why. A developer reading only the schema (without the docs) sees the discrepancy without understanding the rationale.

Fix: Add notes annotations on affected slots explaining the override rationale (BFO alignment, OWL DL annotation property conflict avoidance, semantic precision).