GH554: Improve pkeyutl doc

Signed-off-by: NRich Salz <rsalz@openssl.org>
Reviewed-by: NDr. Stephen Henson <steve@openssl.org>

doc/apps/pkeyutl.pod

@@ -137,6 +137,19 @@ Unless otherwise mentioned all algorithms support the B<digest:alg> option
 which specifies the digest in use for sign, verify and verifyrecover operations.
 The value B<alg> should represent a digest name as used in the
 EVP_get_digestbyname() function for example B<sha1>.
+This value is used only for sanity-checking the lengths of data passed in to
+the B<pkeyutl> and for creating the structures that make up the signature
+(e.g. B<DigestInfo> in RSASSA PKCS#1 v1.5 signatures).
+In case of RSA, ECDSA and DSA signatures, this utility
+will not perform hashing on input data but rather use the data directly as
+input of signature algorithm. Depending on key type, signature type and mode
+of padding, the maximum acceptable lengths of input data differ. In general,
+with RSA the signed data can't be longer than the key modulus, in case of ECDSA
+and DSA the data shouldn't be longer than field size, otherwise it will be
+silently truncated to field size.
+
+In other words, if the value of digest is B<sha1> the input should be 20 bytes
+long binary encoding of SHA-1 hash function output.
 
 =head1 RSA ALGORITHM