Update DH parameter docs.

doc/crypto/d2i_DHparams.pod

@@ -2,7 +2,7 @@
 
 =head1 NAME
 
-d2i_DHparams, i2d_DHparams - ...
+d2i_DHparams, i2d_DHparams - PKCS#3 DH parameter functions.
 
 =head1 SYNOPSIS
 
@@ -13,18 +13,18 @@ d2i_DHparams, i2d_DHparams - ...
 
 =head1 DESCRIPTION
 
-...
+These functions decode and encode PKCS#3 DH parameters using the
+DHparameter structure described in PKCS#3.
 
-=head1 RETURN VALUES
-
-...
+Othewise these behave in a similar way to d2i_X509() and i2d_X509()
+described in the L<d2i_X509(3)|d2i_X509(3)> manual page.
 
 =head1 SEE ALSO
 
-...
+L<d2i_X509(3)|d2i_X509(3)>
 
 =head1 HISTORY
 
-...
+TBA
 
 =cut