xbps-rindex.8: clarify abs paths, detail -f with -a.
xbps-rindex.8: clarify abs paths, detail -f with -a.

stpx/xbps,ebfe/xbps,datenwolf/xbps,ebfe/xbps,stpx/xbps,datenwolf/xbps,datenwolf/xbps,ebfe/xbps,stpx/xbps