go.mobile/sprite/portable: package docs
go.mobile/sprite/portable: package docs

LGTM=nigeltao
R=nigeltao
CC=golang-codereviews
https://golang.org/cl/162370044

swisspol/mobile,ichu501/mobile,swisspol/mobile,samanalysis/mobile,ppamorim/mobile,b54898533/mobile,freeman0211/mobile,zach-klippenstein/mobile,grategames/mobile,swisspol/mobile,lonphy23t/mobile,lonphy23t/mobile,golang/mobile,steeve/mobile,ichu501/mobile,grategames/mobile,DanielWorld/mobile,cainiaocome/mobile,goldcaddy77/mobile,golang/mobile,qskycolor/mobile,golang/mobile,qskycolor/mobile,b54898533/mobile,cainiaocome/mobile,cainiaocome/mobile,0x90sled/mobile,ppamorim/mobile,tempbottle/mobile,ichu501/mobile,MaxCy330/mobile,xing/gomobile,tempbottle/mobile,xing/gomobile,DanielWorld/mobile,DanielWorld/mobile,ppamorim/mobile,steeve/mobile,zach-klippenstein/mobile,freeman0211/mobile,lonphy23t/mobile,MaxCy330/mobile,ichu501/mobile,freeman0211/mobile,b54898533/mobile,tempbottle/mobile,MaxCy330/mobile,freeman0211/mobile,steeve/mobile,lonphy23t/mobile,0x90sled/mobile,zach-klippenstein/mobile,steeve/mobile,ichu501/mobile,samanalysis/mobile,0x90sled/mobile,cainiaocome/mobile,swisspol/mobile,0x90sled/mobile,goldcaddy77/mobile,goldcaddy77/mobile,swisspol/mobile,tempbottle/mobile,grategames/mobile,zach-klippenstein/mobile,goldcaddy77/mobile,xing/gomobile,lonphy23t/mobile,samanalysis/mobile,MaxCy330/mobile,qskycolor/mobile,xing/gomobile,samanalysis/mobile,b54898533/mobile,ppamorim/mobile,goldcaddy77/mobile,golang/mobile,0x90sled/mobile,DanielWorld/mobile,xing/gomobile,MaxCy330/mobile,qskycolor/mobile,cainiaocome/mobile,samanalysis/mobile,ppamorim/mobile,grategames/mobile,tempbottle/mobile,b54898533/mobile