keyctl_dh_compute.3: mention /proc/crypto for hashnames

The kernel does not have a set of known hashnames available. Instead,
/proc/crypto contains the information for the running system.

Signed-off-by: Ben Boeckel <mathstuf@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>
diff --git a/man/keyctl_dh_compute.3 b/man/keyctl_dh_compute.3
index 837c481..5b79378 100644
--- a/man/keyctl_dh_compute.3
+++ b/man/keyctl_dh_compute.3
@@ -89,7 +89,9 @@
 for the key derivation function, such as sha256.
 The
 .I hashname
-must be a NULL terminated string.
+must be a NULL terminated string.  See
+.I /proc/crypto
+for available hashes on the system.
 .P
 Following the specification of SP800-56A section 5.8.1.2 the
 .I otherinfo