In Sarah Polley's unconventional documentary, "Stories We Tell," "truth" is a relative term when family secrets are involved. The Canadian actress-writer-director's quest is resolving her parentage. Did she have another father, as childhood teasing suggested, in addition to the beloved Michael Polley, who raised her and thought her his own? Her mother, Diane, died when Polley was 11. Had she lived, perhaps Polley would have had the answer long ago and the film left uncontemplated. In Diane's absence, everyone has opinions — family members, her mother's friends and lovers. Polley gently, but insistently, grills them all. What surfaces between the lines is memory's complicated skein filled with inconsistencies and conjecture. It is Michael who anchors the film as much as Diane's secrets. As narrator, his moving musings about Polley's discoveries form a portrait too — that of a father's infinite love.