math code: Add comments. * lib/acosl.c: Add comment about related glibc source files. * lib/asinl.c: Likewise. * lib/atanl.c: Likewise. * lib/expl.c: Likewise. * lib/logl.c: Likewise. * lib/sincosl.c: Likewise. * lib/sinl.c: Likewise. * lib/tanl.c: Likewise. * lib/trigl.c: Likewise. * lib/cosl.c: Likewise. Fix comments.