document scalar clamping of curve25519 keys

The "scalars" are just random bytes. To make them secure curve25519
keys, they need to be clamped according to rfc7748 section 5.
As this is not obvious, we need to add a reference to the RFC.

No functional change.

Closes: #324

src/core/utils/x25519.ts

@@ -3,6 +3,8 @@ import { x25519 } from "@noble/curves/ed25519";
 export function getX25519PrivateKey(): Uint8Array {
   const key = x25519.utils.randomPrivateKey();
 
+  // scalar clamping for curve25519, according to
+  // https://www.rfc-editor.org/rfc/rfc7748#section-5
   key[0] &= 248;
   key[31] &= 127;
   key[31] |= 64;