doc/go1.12: mention heap sampling change

This is about a minor change but worthy of note because this
may affect the profile results users will see.

Change-Id: Ie2c4358b248f868662dbc71db587576481aa7238
Reviewed-on: https://go-review.googlesource.com/c/158577
Reviewed-by: Raul Silvera <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

Author: hyangah

doc/go1.12.html

@@ -329,6 +329,11 @@ <h2 id="runtime">Runtime</h2>
   yet supported on Windows.
 </p>
 
+<p><!-- CL 158337 -->
+  Go 1.12 improves the accuracy of memory profiles by fixing
+  overcounting of large heap allocations.
+</p>
+
 <h2 id="library">Core library</h2>
 
 <h3 id="tls_1_3">TLS 1.3</h3>