Remove mention of extract/expand operations from ConcatKDF docs.
Remove mention of extract/expand operations from ConcatKDF docs.

Hasimir/cryptography,sholsapp/cryptography,sholsapp/cryptography,sholsapp/cryptography,Hasimir/cryptography,kimvais/cryptography,Hasimir/cryptography,Ayrx/cryptography,Hasimir/cryptography,Ayrx/cryptography,Ayrx/cryptography,sholsapp/cryptography,Ayrx/cryptography,kimvais/cryptography,kimvais/cryptography,kimvais/cryptography