Add explaining comment about why we have all these OPTIONAL statements for session attributes

[surchs]

We have these optional statements in the default query

api/app/api/utility.py

Lines 252 to 257 in e1a2707

	OPTIONAL {{?dataset_uuid nb:hasPortalURI ?dataset_portal_uri.}}
	OPTIONAL {{?session nb:hasAge ?age.}}
	OPTIONAL {{?session nb:hasSex ?sex.}}
	OPTIONAL {{?session nb:hasDiagnosis ?diagnosis.}}
	OPTIONAL {{?session nb:isSubjectGroup ?subject_group.}}
	OPTIONAL {{?session nb:hasAssessment ?assessment.}}

They are (most likely) required, in order for the query to still match subjects who do not have phenotypic sessions with these atributes. @alyssadai, if you find he PR where we introduced the OPIONAL statements and / or can provide the reason for why they are there, please add it here.

Since we're already a bit confused about them, we should add an explainin comment above the query template string in the code that explains why they are there.