Docs: clarify fromSecretKey.

README.md

@@ -210,14 +210,16 @@ Generates new random key pair for signing and returns it as an object with
 
 #### nacl.sign.keyPair.fromSecretKey(secretKey)
 
-Returns a signing key pair with public key corresponding to the given secret key.
+Returns a signing key pair with public key corresponding to the given
+64-byte secret key. The secret key must have been generated by
+`nacl.sign.keyPair` or `nacl.sign.keyPair.fromSeed`.
 
 #### nacl.sign.keyPair.fromSeed(seed)
 
-Returns a new key pair generated deterministically from a 32-byte seed.  Note
-that seed must contain enough entropy. This method is not recommended for
-general use: instead, use `nacl.sign.keyPair()` to generate a new key pair from
-a random seed.
+Returns a new signing key pair generated deterministically from a 32-byte seed.
+The seed must contain enough entropy to be secure. This method is not
+recommended for general use: instead, use `nacl.sign.keyPair` to generate a new
+key pair from a random seed.
 
 #### nacl.sign(message, secretKey)