doc/go1.19: improve the loong64 release notes
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 #46229
For #51400

Change-Id: I6789f509263a0dbf113481148665e7951aa6a989
Reviewed-on: https://go-review.googlesource.com/c/go/+/417715
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Auto-Submit: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Reviewed-by: abner chenc <a48efcc585c63e6cca478bbbc28c4768d7c43c60@loongson.cn>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go