Reference sampling doc from heapprofd docs.

Change-Id: Ib119c02ece2072c26b9a9d7beb3e03f9134f1457
diff --git a/docs/data-sources/native-heap-profiler.md b/docs/data-sources/native-heap-profiler.md
index ca9e6c8..108a32d 100644
--- a/docs/data-sources/native-heap-profiler.md
+++ b/docs/data-sources/native-heap-profiler.md
@@ -116,6 +116,8 @@
 gets attributed the complete n bytes. For more accuracy, allocations larger than
 the sampling interval bypass the sampling logic and are recorded with their true
 size.
+See the [heapprofd Sampling](/docs/design-docs/heapprofd-sampling) document for
+details.
 
 ## Startup profiling