Document preferences for EC_GROUP_new_by_curve_name.

Folks should use curve25519 or P-256 if in doubt.

Change-Id: Ie35381ef739744788a80345286f7b21e2bb67c88
Reviewed-on: https://boringssl-review.googlesource.com/26646
Commit-Queue: David Benjamin <davidben@google.com>
Commit-Queue: Adam Langley <agl@google.com>
Reviewed-by: Adam Langley <agl@google.com>
CQ-Verified: CQ bot account: commit-bot@chromium.org <commit-bot@chromium.org>

include/openssl/ec.h

diff --git a/include/openssl/ec.h b/include/openssl/ec.h
index fd35b03..5eee366 100644
--- a/include/openssl/ec.h
+++ b/include/openssl/ec.h
@@ -105,10 +105,13 @@
 // curve specified by |nid|, or NULL on error.
 //
 // The supported NIDs are:
-//   NID_secp224r1,
-//   NID_X9_62_prime256v1,
-//   NID_secp384r1,
-//   NID_secp521r1
+//   NID_secp224r1 (P-224),
+//   NID_X9_62_prime256v1 (P-256),
+//   NID_secp384r1 (P-384),
+//   NID_secp521r1 (P-521)
+//
+// If in doubt, use |NID_X9_62_prime256v1|, or see the curve25519.h header for
+// more modern primitives.
 OPENSSL_EXPORT EC_GROUP *EC_GROUP_new_by_curve_name(int nid);
 
 // EC_GROUP_free frees |group| and the data that it points to.