ECDSA_SIG: restore doc comments which were deleted accidentally

amends 0396401d1c3f

Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/6366)
diff --git a/include/openssl/ec.h b/include/openssl/ec.h
index a24bee0..ed2161d 100644
--- a/include/openssl/ec.h
+++ b/include/openssl/ec.h
@@ -1078,6 +1078,8 @@
 
 /** Setter for r and s fields of ECDSA_SIG
  *  \param  sig  pointer to ECDSA_SIG structure
+ *  \param  r    pointer to BIGNUM for r (may be NULL)
+ *  \param  s    pointer to BIGNUM for s (may be NULL)
  */
 int ECDSA_SIG_set0(ECDSA_SIG *sig, BIGNUM *r, BIGNUM *s);