AMP-Cache-Transform: explain caches registry. (#18495)
AMP-Cache-Transform: explain caches registry. (#18495)

Briefly explain the reasoning behind limiting subresource URLs to those
under caches.json, and add a stricter requirement for JS URLs, given
their stronger security relevance.
mhc-gh/amphtml,lannka/amphtml,rsimha-amp/amphtml,sharethrough/amphtml,media-net/amphtml,laterpay/amphtml,prateekbh/amphtml,sharethrough/amphtml,lannka/amphtml,zhouyx/amphtml,laterpay/amphtml,zhouyx/amphtml,alanorozco/amphtml,voyagegroup/amphtml,lannka/amphtml,jridgewell/amphtml,sharethrough/amphtml,cory-work/amphtml,alanorozco/amphtml,jridgewell/amphtml,zhouyx/amphtml,sharethrough/amphtml,yandex-pcode/amphtml,src-code/amphtml,adup-tech/amphtml,yandex-pcode/amphtml,techhtml/amphtml,BenDz/amphtml,danielrozenberg/amphtml,cvializ/amphtml,BenDz/amphtml,koshka/amphtml,prateekbh/amphtml,honeybadgerdontcare/amphtml,danielrozenberg/amphtml,koshka/amphtml,nexxtv/amphtml,jpettitt/amphtml,Gregable/amphtml,rsimha-amp/amphtml,smartadserver/amphtml,erwinmombay/amphtml,sebastianbenz/amphtml,src-code/amphtml,lannka/amphtml,alanorozco/amphtml,media-net/amphtml,techhtml/amphtml,cvializ/amphtml,dotandads/amphtml,honeybadgerdontcare/amphtml,alanorozco/amphtml,jmadler/amphtml,pedjoni8/amphtml,cory-work/amphtml,dotandads/amphtml,zhangsu/amphtml,danielrozenberg/amphtml,cvializ/amphtml,koshka/amphtml,widespace-os/amphtml,tlong2/amphtml,adup-tech/amphtml,laterpay/amphtml,kamtschatka/amphtml,nexxtv/amphtml,cvializ/amphtml,Gregable/amphtml,cory-work/amphtml,pedjoni8/amphtml,widespace-os/amphtml,ampproject/amphtml,widespace-os/amphtml,BenDz/amphtml,laterpay/amphtml,Gregable/amphtml,mhc-gh/amphtml,laterpay/amphtml,mhc-gh/amphtml,yandex-pcode/amphtml,jmadler/amphtml,prateekbh/amphtml,Gregable/amphtml,adup-tech/amphtml,adup-tech/amphtml,yieldbot/amphtml,ampproject/amphtml,alanorozco/amphtml,nexxtv/amphtml,media-net/amphtml,voyagegroup/amphtml,adup-tech/amphtml,tlong2/amphtml,jmadler/amphtml,honeybadgerdontcare/amphtml,mhc-gh/amphtml,voyagegroup/amphtml,erwinmombay/amphtml,widespace-os/amphtml,src-code/amphtml,lannka/amphtml,laterpay/amphtml,taboola/amphtml,danielrozenberg/amphtml,koshka/amphtml,honeybadgerdontcare/amphtml,jridgewell/amphtml,erwinmombay/amphtml,dotandads/amphtml,sharethrough/amphtml,lannka/amphtml,sebastianbenz/amphtml,yandex-pcode/amphtml,BenDz/amphtml,tlong2/amphtml,adup-tech/amphtml,danielrozenberg/amphtml,techhtml/amphtml,jpettitt/amphtml,alanorozco/amphtml,nexxtv/amphtml,kamtschatka/amphtml,yandex-pcode/amphtml,tlong2/amphtml,zhouyx/amphtml,jmadler/amphtml,Gregable/amphtml,jridgewell/amphtml,yandex-pcode/amphtml,Gregable/amphtml,sebastianbenz/amphtml,zhangsu/amphtml,rsimha-amp/amphtml,prateekbh/amphtml,ampproject/amphtml,nexxtv/amphtml,zhangsu/amphtml,jmadler/amphtml,media-net/amphtml,alanorozco/amphtml,nexxtv/amphtml,kamtschatka/amphtml,BenDz/amphtml,zhangsu/amphtml,cory-work/amphtml,jridgewell/amphtml,cvializ/amphtml,zhangsu/amphtml,media-net/amphtml,smartadserver/amphtml,src-code/amphtml,jpettitt/amphtml,kamtschatka/amphtml,zhouyx/amphtml,yieldbot/amphtml,taboola/amphtml,rsimha-amp/amphtml,voyagegroup/amphtml,yandex-pcode/amphtml,lannka/amphtml,kamtschatka/amphtml,yieldbot/amphtml,taboola/amphtml,danielrozenberg/amphtml,cory-work/amphtml,pedjoni8/amphtml,smartadserver/amphtml,honeybadgerdontcare/amphtml,prateekbh/amphtml,ampproject/amphtml,cory-work/amphtml,techhtml/amphtml,honeybadgerdontcare/amphtml,tlong2/amphtml,taboola/amphtml,dotandads/amphtml,techhtml/amphtml,techhtml/amphtml,pedjoni8/amphtml,ampproject/amphtml,sebastianbenz/amphtml,jpettitt/amphtml,media-net/amphtml,rsimha-amp/amphtml,tlong2/amphtml,erwinmombay/amphtml,yieldbot/amphtml,sebastianbenz/amphtml,jridgewell/amphtml,pedjoni8/amphtml,danielrozenberg/amphtml,nexxtv/amphtml,zhouyx/amphtml,mhc-gh/amphtml,cvializ/amphtml,erwinmombay/amphtml,prateekbh/amphtml,mhc-gh/amphtml,src-code/amphtml,kamtschatka/amphtml,taboola/amphtml,smartadserver/amphtml,zhangsu/amphtml,jridgewell/amphtml,voyagegroup/amphtml,jpettitt/amphtml,media-net/amphtml,smartadserver/amphtml,prateekbh/amphtml,widespace-os/amphtml,mhc-gh/amphtml,cory-work/amphtml,taboola/amphtml,erwinmombay/amphtml,smartadserver/amphtml,yieldbot/amphtml,BenDz/amphtml,voyagegroup/amphtml,zhouyx/amphtml,sebastianbenz/amphtml,koshka/amphtml,jmadler/amphtml,koshka/amphtml,danielrozenberg/amphtml,dotandads/amphtml,voyagegroup/amphtml,dotandads/amphtml,alanorozco/amphtml,ampproject/amphtml,media-net/amphtml,Gregable/amphtml,taboola/amphtml,ampproject/amphtml,ampproject/amphtml,src-code/amphtml,yieldbot/amphtml,rsimha-amp/amphtml,jpettitt/amphtml,widespace-os/amphtml,jpettitt/amphtml,jpettitt/amphtml,rsimha-amp/amphtml,sharethrough/amphtml,pedjoni8/amphtml,honeybadgerdontcare/amphtml,mhc-gh/amphtml,Gregable/amphtml