Dion is far from the only celebrity to have invited the public to witness life with a serious illness. Lady Gaga’s 2017 documentary, “Gaga: Five Foot Two,” revealed the star’s daily struggle with fibromyalgia, and in last year’s “Still,” Michael J. Fox — a groundbreaking figure in celebrity-illness transparency — further tugged down the curtain on how severe his Parkinson’s disease has become. Selma Blair, who spent a portion of her career hiding symptoms, eventually revealed a diagnosis of M.S. and then began posting intensely personal bedside updates on social media. Last year an issue of British Vogue had her on the cover, in a skinny beige column of a dress, patent pumps and a cane, with a headline announcing her as “Dynamic, Daring & Disabled.”