EVP_KDF.pod: extend text on 'salt' and 'info' parameters

Reviewed-by: Tomas Mraz <tomas@openssl.org>
Reviewed-by: Paul Dale <pauli@openssl.org>
(Merged from #21469)

(cherry picked from commit 61c8146)

doc/man3/EVP_KDF.pod

@@ -191,7 +191,7 @@ For those KDF implementations that support it, this parameter sets the password.
 
 =item "salt" (B<OSSL_KDF_PARAM_SALT>) <octet string>
 
-Some KDF implementations can take a salt.
+Some KDF implementations can take a non-secret unique cryptographic salt.
 For those KDF implementations that support it, this parameter sets the salt.
 
 The default value, if any, is implementation dependent.
@@ -227,6 +227,15 @@ Some KDF implementations require a key.
 For those KDF implementations that support it, this octet string parameter
 sets the key.
 
+=item "info" (B<OSSL_KDF_PARAM_INFO>) <octet string>
+
+Some KDF implementations, such as L<EVP_KDF-HKDF(7)>, take an 'info' parameter
+for binding the derived key material
+to application- and context-specific information.
+This parameter sets the info, fixed info, other info or shared info argument.
+You can specify this parameter multiple times, and each instance will
+be concatenated to form the final value.
+
 =item "maclen" (B<OSSL_KDF_PARAM_MAC_SIZE>) <unsigned integer>
 
 Used by implementations that use a MAC with a variable output size (KMAC).