Fix various missing option help messages ...

Reviewed-by: Andy Polyakov <appro@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1585)
diff --git a/apps/pkcs8.c b/apps/pkcs8.c
index 22b5866..3592a0f 100644
--- a/apps/pkcs8.c
+++ b/apps/pkcs8.c
@@ -38,7 +38,7 @@
     {"nocrypt", OPT_NOCRYPT, '-', "Use or expect unencrypted private key"},
     {"v2", OPT_V2, 's', "Use PKCS#5 v2.0 and cipher"},
     {"v1", OPT_V1, 's', "Use PKCS#5 v1.5 and cipher"},
-    {"v2prf", OPT_V2PRF, 's'},
+    {"v2prf", OPT_V2PRF, 's', "Set the PRF algorithm to use with PKCS#5 v2.0"},
     {"iter", OPT_ITER, 'p', "Specify the iteration count"},
     {"passin", OPT_PASSIN, 's', "Input file pass phrase source"},
     {"passout", OPT_PASSOUT, 's', "Output file pass phrase source"},