Clean-up and document refinement block location.
Clean-up and document refinement block location.

Adds some documentation for what's going on in the code that
 identifies which blocks are affected by a given DP node during MV
 refinement.

ascent12/daala,xiphmont/daala,HeadhunterXamd/daala,KyleSiefring/daala,luctrudeau/daala,KyleSiefring/daala,ycho/daala,xiph/daala,felipebetancur/daala,jmvalin/daala,xiphmont/daala,xiphmont/daala,nvoron23/daala,iankronquist/daala,xiph/daala,felipebetancur/daala,luctrudeau/daala,KyleSiefring/daala,jmvalin/daala,kodabb/daala,mbebenita/daala,kodabb/daala,kustom666/daala,ascent12/daala,xiph/daala,KyleSiefring/daala,xiph/daala,ascent12/daala,tribouille/daala,luctrudeau/daala,mbebenita/daala,mbebenita/daala,ycho/daala,xiph/daala,mbebenita/daala,felipebetancur/daala,KyleSiefring/daala,HeadhunterXamd/daala,HeadhunterXamd/daala,iankronquist/daala,kodabb/daala,kustom666/daala,HeadhunterXamd/daala,luctrudeau/daala,nvoron23/daala,iankronquist/daala,jmvalin/daala,felipebetancur/daala,kustom666/daala,tribouille/daala,xiphmont/daala,tribouille/daala,luctrudeau/daala,iankronquist/daala,iankronquist/daala,HeadhunterXamd/daala,xiphmont/daala,nvoron23/daala,ascent12/daala,jmvalin/daala,kustom666/daala,tribouille/daala,nvoron23/daala,ycho/daala,ascent12/daala,mbebenita/daala,kustom666/daala,kodabb/daala,felipebetancur/daala,nvoron23/daala,ycho/daala,kodabb/daala,jmvalin/daala,ycho/daala,tribouille/daala