Finish documenting asn1.h.

One down, two more to go! As part of this, I've added it to doc.config,
revised the note at the top, and moved the sample i2d/d2i functions
here.

Bug: 426
Change-Id: I7bb9d56bf9ba58c921cfcf9626bf3647c6e5c7df
Reviewed-on: https://boringssl-review.googlesource.com/c/boringssl/+/50107
Reviewed-by: Adam Langley <agl@google.com>
Commit-Queue: David Benjamin <davidben@google.com>
diff --git a/util/doc.config b/util/doc.config
index aacedea..12fdefc 100644
--- a/util/doc.config
+++ b/util/doc.config
@@ -52,6 +52,11 @@
       "include/openssl/hpke.h"
     ]
   },{
+    "Name": "Legacy ASN.1 and X.509 implementation (documentation in progress)",
+    "Headers": [
+      "include/openssl/asn1.h"
+    ]
+  },{
     "Name": "SSL implementation",
     "Headers": [
       "include/openssl/ssl.h"