[newlib-cygwin/main] Cygwin: add release note for newgrp(1) fixes

[Corinna Vinschen]

https://sourceware.org/git/gitweb.cgi?p=newlib-cygwin.git;h=43ddce194c496f81519b4c5cdba0dd39eea0edce

commit 43ddce194c496f81519b4c5cdba0dd39eea0edce
Author:     Corinna Vinschen <corinna@vinschen.de>
AuthorDate: Fri Dec 5 20:39:11 2025 +0100
Commit:     Corinna Vinschen <corinna@vinschen.de>
CommitDate: Wed Dec 10 18:10:10 2025 +0100

    Cygwin: add release note for newgrp(1) fixes
    
    Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

Diff:
---
 winsup/cygwin/release/3.6.6 | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/winsup/cygwin/release/3.6.6 b/winsup/cygwin/release/3.6.6
index 09b6a3913df4..194ff15d0b84 100644
--- a/winsup/cygwin/release/3.6.6
+++ b/winsup/cygwin/release/3.6.6
@@ -14,3 +14,6 @@ Fixes:
 
 - Fix the problem that tmp_pathbuf area is destroyed regarding flock().
   Addresses: https://cygwin.com/pipermail/cygwin/2025-October/258914.html
+
+- Improve newgrp(1) POSIX compatibility and its documentation.
+  Addresses: https://cygwin.com/pipermail/cygwin/2025-December/259055.html