_content/doc/go1.27: document syscall.Errno on Plan 9

CL 750680 defined the syscall.Errno type on Plan 9,
and CL 751100 made it implement the error interface.
syscall.Errno is now available on Plan 9, as on the
other systems, so portable code can refer to it
without build constraints.

Add a release note for the syscall package.

For golang/go#78779.

Change-Id: Ib06781dd843d0e986cb33f64796e578d45a7c0d8
Reviewed-on: https://go-review.googlesource.com/c/website/+/796360
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Hongxiang Jiang <hxjiang@golang.org>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

_content/doc/go1.27.md

diff --git a/_content/doc/go1.27.md b/_content/doc/go1.27.md
index 47381b8..202f16f 100644
--- a/_content/doc/go1.27.md
+++ b/_content/doc/go1.27.md
@@ -608,6 +608,16 @@
 around the last occurrence of a separator.
 It can replace and simplify some common uses of [`LastIndex`](/pkg/strings#LastIndex).
 
+#### [`syscall`](/pkg/syscall/)
+
+<!-- CL 750680 -->
+
+On Plan 9, the [`Errno`](/pkg/syscall#Errno) type is now defined and
+implements the `error` interface, as on other platforms. Plan 9 system
+calls return `ErrorString` values, so `Errno` is never returned by this
+package on Plan 9. It is defined so that portable code referring to
+`syscall.Errno` builds on Plan 9 without build constraints.
+
 #### [`testing/synctest`](/pkg/testing/synctest/)
 
 <!-- 6-stdlib/99-minor/testing/synctest/77169.md -->