update docs to explain CTR generation source
update docs to explain CTR generation source

Lukasa/cryptography,kimvais/cryptography,dstufft/cryptography,kimvais/cryptography,dstufft/cryptography,Ayrx/cryptography,bwhmather/cryptography,Hasimir/cryptography,sholsapp/cryptography,bwhmather/cryptography,Hasimir/cryptography,Lukasa/cryptography,skeuomorf/cryptography,Ayrx/cryptography,skeuomorf/cryptography,skeuomorf/cryptography,bwhmather/cryptography,sholsapp/cryptography,Lukasa/cryptography,bwhmather/cryptography,Ayrx/cryptography,dstufft/cryptography,kimvais/cryptography,Hasimir/cryptography,Hasimir/cryptography,sholsapp/cryptography,sholsapp/cryptography,Ayrx/cryptography,kimvais/cryptography,skeuomorf/cryptography,dstufft/cryptography,dstufft/cryptography