doc/go1.12: document change in syscall.Setrlimit behavior

Fixes #30401

Change-Id: I7b5035ffc7333c746d4e31563df26ff4f934dfc6
Reviewed-on: https://go-review.googlesource.com/c/go/+/188237
Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
Reviewed-by: Keith Randall <khr@golang.org>

doc/go1.12.html

@@ -80,6 +80,10 @@ <h3 id="darwin">Darwin</h3>
   checks for private API usage. Since it is considered private,
   <code>syscall.Getdirentries</code> now always fails with
   <code>ENOSYS</code> on iOS.
+  Additionally, <a href="/pkg/syscall/#Setrlimit"><code>syscall.Setrlimit</code></a>
+  reports <code>invalid</code> <code>argument</code> in places where it historically
+  succeeded. These consequences are not specific to Go and users should expect
+  behavioral parity with <code>libSystem</code>'s implementation going forward.
 </p>
 
 <h2 id="tools">Tools</h2>