Penelope Spheeris' documentary of the L.A. punk rock scene circa '79-'80 does occasionally threaten to come across as the effort of a superfan explaining that newfangled noise to fearful old fogeys who had been locking their daughters away from the mohawks and safety pins brigade, but thankfully she's smart enough to resist narrating and let the scene do the talking. Bizarrely it gives the documentary a weird, pacifying mix of authenticity and danger that gives it cred while assuaging the…