dma.8: fix problems reported by igor and 'mandoc -Tlint'

dma.8:77:contraction:Queue the mail, but [don't] attempt to deliver it.
dma.8:85:repeated:s [are are] ignored.
dma.8:87:contraction:[Don't] run in the background.
dma.8:201:contraction:Use the catch-all alias only if you [don't] want any local mail to be

mandoc: dma.8:308:5: WARNING: macro neither callable nor escaped: Sm

MFC after: 3 days