STAMFORD — An apparent misunderstanding by family and friends has fueled a social media firestorm over police response to an 18-year-old Stamford man who prompted the lockdown of an elementary school this week. A woman on Twitter who identified herself as the man’s sister posted a photo of him with a neck brace and a cut on his forehead, alleging a case of brutality by city police, who took the man into custody behind Rogers International School on Monday morning following several 911 calls about a man acting irrationally outside the elementary school.