Here's some prose you might want to use or edit.

```
A salt. Randomizes the KDF's output. Optional, but highly recommended.
Ideally as many bits of entropy as the security level of the hash:
often that means cryptographically random and as long as the hash
output. Worse (shorter, less entropy) salt values can still
meaningfully contribute to security. May be reused. Does not have to
be secret, but may cause stronger security guarantees if secret; see
RFC 5869 and the HKDF paper for more details.
```

References: 
RFC: https://tools.ietf.org/html/rfc5869
Paper: http://eprint.iacr.org/2010/264