Put some docs for default_backend in a place.
Put some docs for default_backend in a place.

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