New York (tca/dpa) — “This is by far the biggest crowd I’ve had in a few years,” said Celine Dion onstage at Lincoln Center last week. She was making a rare appearance to introduce “I Am: Celine Dion,” a documentary chronicling her struggles with stiff person syndrome, a rare neurological disorder that causes muscle rigidity and has made it difficult for her to do the thing that has most defined her since childhood: sing. “I cannot believe how fortunate I am to have my fans in my life,” Dion said, pausing to hold back tears as her son, René-Charles Angélil, who was waiting on the side of the s…