doc/go1.18: document that iOS 12 or newer is required

For #47694.
Updates #49616.
Updates #48076.

Change-Id: I570564c3a54d3cd9cfc9b8267df9fbee3363b650
Reviewed-on: https://go-review.googlesource.com/c/go/+/366914
Trust: Dmitri Shuralyov <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>

Author: dmitshur

doc/go1.18.html

@@ -105,6 +105,14 @@ <h3 id="riscv">RISC-V</h3>
   now supports the <code>c-archive</code> and <code>c-shared</code> build modes.
 </p>
 
+<h3 id="ios">iOS</h3>
+
+<p><!-- golang.org/issue/48076, golang.org/issue/49616 -->
+  On iOS (the <code>ios/arm64</code> port)
+  and iOS simulator running on AMD64-based macOS (the <code>ios/amd64</code> port),
+  Go 1.18 now requires iOS 12 or later; support for previous versions has been discontinued.
+</p>
+
 <h2 id="tools">Tools</h2>
 
 <h3 id="go-command">Go command</h3>