usage.dox: fix doxygen warnings in 1.8.x
usage.dox: fix doxygen warnings in 1.8.x

use \li to denote list items with \if.

fixes the following likely visible in <1.8.3:
usage.dox: warning: Invalid list item found
usage.dox: warning: End of list marker found without any preceding list items

Change-Id: I33c72799edf9f8866596ac8f79247050b8c75681

felipebetancur/libvpx,Suvarna1488/webm.libvpx,zofuthan/libvpx,kim42083/webm.libvpx,altogother/webm.libvpx,stewnorriss/libvpx,webmproject/libvpx,Topopiccione/libvpx,mwgoldsmith/libvpx,iniwf/webm.libvpx,reimaginemedia/webm.libvpx,abwiz0086/webm.libvpx,Topopiccione/libvpx,shacklettbp/aom,running770/libvpx,kalli123/webm.libvpx,felipebetancur/libvpx,mbebenita/aom,matanbs/webm.libvpx,Suvarna1488/webm.libvpx,running770/libvpx,kim42083/webm.libvpx,jacklicn/webm.libvpx,iniwf/webm.libvpx,VTCSecureLLC/libvpx,matanbs/webm.libvpx,Maria1099/webm.libvpx,felipebetancur/libvpx,mwgoldsmith/libvpx,VTCSecureLLC/libvpx,lyx2014/libvpx_c,mwgoldsmith/vpx,mbebenita/aom,lyx2014/libvpx_c,Suvarna1488/webm.libvpx,ShiftMediaProject/libvpx,jacklicn/webm.libvpx,jmvalin/aom,running770/libvpx,altogother/webm.libvpx,Distrotech/libvpx,webmproject/libvpx,Suvarna1488/webm.libvpx,liqianggao/libvpx,matanbs/webm.libvpx,jmvalin/aom,goodleixiao/vpx,reimaginemedia/webm.libvpx,openpeer/libvpx_new,abwiz0086/webm.libvpx,thdav/aom,stewnorriss/libvpx,openpeer/libvpx_new,luctrudeau/aom,mbebenita/aom,stewnorriss/libvpx,reimaginemedia/webm.libvpx,running770/libvpx,kalli123/webm.libvpx,jmvalin/aom,jacklicn/webm.libvpx,smarter/aom,smarter/aom,stewnorriss/libvpx,ittiamvpx/libvpx-1,kalli123/webm.libvpx,mwgoldsmith/vpx,webmproject/libvpx,kleopatra999/webm.libvpx,iniwf/webm.libvpx,ShiftMediaProject/libvpx,Acidburn0zzz/webm.libvpx,Distrotech/libvpx,openpeer/libvpx_new,goodleixiao/vpx,liqianggao/libvpx,kalli123/webm.libvpx,Suvarna1488/webm.libvpx,Suvarna1488/webm.libvpx,liqianggao/libvpx,mbebenita/aom,kim42083/webm.libvpx,mbebenita/aom,iniwf/webm.libvpx,thdav/aom,gshORTON/webm.libvpx,mwgoldsmith/libvpx,lyx2014/libvpx_c,Distrotech/libvpx,Maria1099/webm.libvpx,goodleixiao/vpx,Topopiccione/libvpx,kleopatra999/webm.libvpx,VTCSecureLLC/libvpx,thdav/aom,Acidburn0zzz/webm.libvpx,goodleixiao/vpx,mwgoldsmith/vpx,lyx2014/libvpx_c,mwgoldsmith/vpx,mbebenita/aom,matanbs/webm.libvpx,Topopiccione/libvpx,Distrotech/libvpx,stewnorriss/libvpx,shareefalis/libvpx,kleopatra999/webm.libvpx,iniwf/webm.libvpx,mbebenita/aom,luctrudeau/aom,mbebenita/aom,stewnorriss/libvpx,matanbs/webm.libvpx,openpeer/libvpx_new,felipebetancur/libvpx,Maria1099/webm.libvpx,shyamalschandra/libvpx,lyx2014/libvpx_c,ShiftMediaProject/libvpx,shyamalschandra/libvpx,reimaginemedia/webm.libvpx,ittiamvpx/libvpx-1,webmproject/libvpx,goodleixiao/vpx,kalli123/webm.libvpx,gshORTON/webm.libvpx,shareefalis/libvpx,shacklettbp/aom,webmproject/libvpx,shyamalschandra/libvpx,jmvalin/aom,Maria1099/webm.libvpx,abwiz0086/webm.libvpx,GrokImageCompression/aom,luctrudeau/aom,ShiftMediaProject/libvpx,jacklicn/webm.libvpx,altogother/webm.libvpx,altogother/webm.libvpx,shareefalis/libvpx,openpeer/libvpx_new,thdav/aom,ittiamvpx/libvpx-1,kleopatra999/webm.libvpx,Acidburn0zzz/webm.libvpx,running770/libvpx,ittiamvpx/libvpx-1,Maria1099/webm.libvpx,shareefalis/libvpx,Distrotech/libvpx,Topopiccione/libvpx,abwiz0086/webm.libvpx,luctrudeau/aom,shyamalschandra/libvpx,Acidburn0zzz/webm.libvpx,goodleixiao/vpx,Topopiccione/libvpx,zofuthan/libvpx,gshORTON/webm.libvpx,zofuthan/libvpx,mwgoldsmith/libvpx,openpeer/libvpx_new,reimaginemedia/webm.libvpx,liqianggao/libvpx,smarter/aom,kleopatra999/webm.libvpx,gshORTON/webm.libvpx,kim42083/webm.libvpx,luctrudeau/aom,GrokImageCompression/aom,shareefalis/libvpx,liqianggao/libvpx,Maria1099/webm.libvpx,thdav/aom,gshORTON/webm.libvpx,shacklettbp/aom,running770/libvpx,mwgoldsmith/vpx,kim42083/webm.libvpx,ShiftMediaProject/libvpx,shyamalschandra/libvpx,VTCSecureLLC/libvpx,GrokImageCompression/aom,GrokImageCompression/aom,zofuthan/libvpx,zofuthan/libvpx,gshORTON/webm.libvpx,altogother/webm.libvpx,kleopatra999/webm.libvpx,jmvalin/aom,mwgoldsmith/vpx,iniwf/webm.libvpx,zofuthan/libvpx,GrokImageCompression/aom,mbebenita/aom,matanbs/webm.libvpx,luctrudeau/aom,kalli123/webm.libvpx,webmproject/libvpx,kim42083/webm.libvpx,Acidburn0zzz/webm.libvpx,felipebetancur/libvpx,shyamalschandra/libvpx,smarter/aom,Distrotech/libvpx,smarter/aom,ittiamvpx/libvpx-1,liqianggao/libvpx,abwiz0086/webm.libvpx,shacklettbp/aom,altogother/webm.libvpx,ittiamvpx/libvpx-1,shareefalis/libvpx,thdav/aom,Acidburn0zzz/webm.libvpx,shacklettbp/aom,mwgoldsmith/libvpx,GrokImageCompression/aom,jacklicn/webm.libvpx,lyx2014/libvpx_c,shacklettbp/aom,VTCSecureLLC/libvpx,VTCSecureLLC/libvpx,jacklicn/webm.libvpx,smarter/aom,abwiz0086/webm.libvpx,reimaginemedia/webm.libvpx,felipebetancur/libvpx,mwgoldsmith/libvpx,jmvalin/aom