doc/go1.19: improve the loong64 release notes

Link to the LoongArch documentations site, mention the ABI variant
supported, and add a note about the unfortunate old-world/new-world split
situation that users must be aware of.

Updates golang#46229
For golang#51400

Change-Id: I6789f509263a0dbf113481148665e7951aa6a989
Reviewed-on: https://go-review.googlesource.com/c/go/+/417715
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: abner chenc <chenguoqi@loongson.cn>

doc/go1.19.html

@@ -44,10 +44,19 @@ <h2 id="mem">Memory Model</h2>
 
 <h2 id="ports">Ports</h2>
 
-<h3 id="loong64">Loongson 64-bit</h3>
+<h3 id="loong64">LoongArch 64-bit</h3>
 <p><!-- https://go.dev/issue/46229 -->
-  Go 1.19 adds support for the Loongson 64-bit architecture LoongArch
+  Go 1.19 adds support for the Loongson 64-bit architecture
+  <a href="https://loongson.github.io/LoongArch-Documentation">LoongArch</a>
   on Linux (<code>GOOS=linux</code>, <code>GOARCH=loong64</code>).
+  The implemented ABI is LP64D. Minimum kernel version supported is 5.19.
+</p>
+<p>
+  Note that most existing commercial Linux distributions for LoongArch come
+  with older kernels, with a historical incompatible system call ABI.
+  Compiled binaries will not work on these systems, even if statically linked.
+  Users on such unsupported systems are limited to the distribution-provided
+  Go package.
 </p>
 
 <h3 id="riscv64">RISC-V</h3>