In r291668, an instruction was added to sigcode64.S without the nop pad at the end being taken out. Due to alignment, this means that a dword is being wasted on the shared page for no reason. Take out this nop, and add some comments while I'm here. Reviewed by: jhibbits Sponsored by: Tag1 Consulting, Inc. Differential Revision: https://reviews.freebsd.org/D23055