doc/go1.16: document riscv64 port changes

For #36641
For #40700

Change-Id: Ib268559a2ce7839372dbf273d95876d8d4521a45
Reviewed-on: https://go-review.googlesource.com/c/go/+/274478
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Joel Sing <joel@sing.id.au>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>

doc/go1.16.html

@@ -81,6 +81,14 @@ <h3 id="386">386</h3>
   with <code>GO386=softfloat</code>.
 </p>
 
+<h3 id="riscv">RISC-V</h3>
+
+<p><!-- golang.org/issue/36641, CL 267317 -->
+  The <code>linux/riscv64</code> port now supports cgo and
+  <code>-buildmode=pie</code>. This release also includes performance
+  optimizations and code generation improvements for RISC-V.
+</p>
+
 <h2 id="tools">Tools</h2>
 
 <p>