docs: remove invalid docsting note about auto-assigned bom-ref values

[jkowalleck]

the auto-assigning UUITv4 was removed in a breaking change in v6.0.0 .

• Object model.bom_ref.BomRef's property value defaults to Null, was arbitrary UUID ([BomRef.value default to None #504] via [BomRef.value default None #505])

this fix aligns the docs.

[codacy-production]

Coverage summary from Codacy

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Coverage variation	Diff coverage
Report missing for 543768e1	✅ ∅ (target: 80.00%)

Coverage variation details

	Coverable lines	Covered lines	Coverage
Common ancestor commit (543768e)	Report Missing	Report Missing	Report Missing
Head commit (ffd1870)	4875	4559	93.52%

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Diff coverage details

	Coverable lines	Covered lines	Diff coverage
Pull request (#733)	0	0	∅ (not applicable)

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