doc/go1.12: link to ABIInternal design document

The ABI changes should be completely transparent to Go code, but could
cause linking issues in certain situations involving assembly code
reaching across package boundaries. If users encounter linking
problems, point them to the "Compatibility" section of the ABI design
document, which gives some guidance.

Change-Id: I4156d164562e2ec0de7ae8f9a3631a32ec45b317
Reviewed-on: https://go-review.googlesource.com/c/158237Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

doc/go1.12.html

@@ -239,9 +239,9 @@ for {
   except for calls that simultaneously cross between Go and
   assembly <em>and</em> cross a package boundary. If linking results
   in an error like "relocation target not defined for ABIInternal (but
-  is defined for ABI0)", please refer to help section of the ABI
-  design document.
-  <!-- TODO(austin): Link to the design doc. -->
+  is defined for ABI0)", please refer to the
+  <a href="https://github.com/golang/proposal/blob/master/design/27539-internal-abi.md#compatibility">compatibility section</a>
+  of the ABI design document.
 </p>
 
 <p><!-- CL 145179 -->