Behind the Shaws on Main Street and down the sloping grass of Second Street park, a low-slung concrete wall cuts along the park’s wooded boundary. It’s referred to as “the wall,” and it has grown into one the city’s more popular outdoor hangouts this summer — mostly for teenagers, but also adults whose drinking and rowdy behavior spurred the police department to increase its patrols of the area following a steady stream of complaints, the department said. The scene at the wall also has triggered some parents to worry about the safety of minors who hang out there, especially with regard to underage drinking and the use of crass language that objectifies girls.