Remove claim that Zbkb is a subset of Zbb (riscv#1660)

Fixes riscv#1658

Author: aswaterman

src/scalar-crypto.adoc

@@ -204,8 +204,9 @@ protocols, while ShangMi ciphers are required for use in China.
 [[zbkb-sc,Zbkb-sc]]
 ==== `Zbkb` - Bitmanip instructions for Cryptography
 
-These are a subset of the Bitmanipulation Extension `Zbb` which are
-particularly useful for Cryptography. Please refer to <<b-st-ext.adoc#zbkb>>.
+This extension contains bit-manipulation instructions that are particularly
+useful for cryptography, most of which are also in the `Zbb` extension.
+Please refer to <<b-st-ext.adoc#zbkb>>.
 
 [[zbkc-sc,Zbkc-sc]]
 ==== `Zbkc` - Carry-less multiply instructions