Mention calling of SQL functions in `Selectable` docs #2991

Job	Run time
Check sqlite bundled + Sqlite with asan	7m 57s
Check Minimal supported rust version (1.66.0)	6m 24s
Check rustfmt style && run clippy	12m 44s
Compiletests	1m 54s
Spell Check with Typos	9s
Check	0s
	29m 8s

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