[glibc] manual: Add stability warning regarding system-wide tunables

https://sourceware.org/git/gitweb.cgi?p=glibc.git;h=7ace1143691919bc63a553440f7399660c367833

commit 7ace1143691919bc63a553440f7399660c367833
Author: Florian Weimer <[email protected]>
Date:   Mon Jul 20 15:53:56 2026 +0200

    manual: Add stability warning regarding system-wide tunables
    
    And mention system-wide tunables in NEWS.
    Reviewed-by: Andreas K. Hüttel <[email protected]>

Diff:
---
 NEWS                 | 5 +++++
 manual/tunables.texi | 8 +++++++-
 2 files changed, 12 insertions(+), 1 deletion(-)

diff --git a/NEWS b/NEWS
index 0217dde5e5..e1c1db284a 100644
--- a/NEWS
+++ b/NEWS
@@ -9,6 +9,11 @@ Version 2.44
 
 Major new features:
 
+* System-wide tunables can be applied using /etc/tunables.conf and
+  running ldconfig.  Specific tunable settings and the
+  /etc/tunables.conf file format and path are not part of the stable
+  library interfaces and may change between releases.
+
 * A new tunable, glibc.elf.thp, is added to map read-only segments with
   Transparent Huge Pages (THP) if THP is not disabled in the kernel.  When
   glibc.elf.thp is set to 1, malloc uses the actual kernel THP mode
diff --git a/manual/tunables.texi b/manual/tunables.texi
index e7dcef1040..b53cfbf4be 100644
--- a/manual/tunables.texi
+++ b/manual/tunables.texi
@@ -92,7 +92,13 @@ In addition to setting the @code{GLIBC_TUNABLES} environment variable,
 tunables may be provided globally via the file
 @file{/etc/tunables.conf}, which gets stored in glibc's dynamic
 library cache @file{/etc/ld.so.cache} and read by every program at
-startup.  @file{/etc/tunables.conf} contains one tunable per line:
+startup.
+
+@strong{Portability Note:} Specific tunable settings and the
+@file{/etc/tunables.conf} file format and path are not part of the
+stable library interfaces and may change between releases.
+
+The file @file{/etc/tunables.conf} contains one tunable per line:
 
 @example
 glibc.malloc.trim_threshold=128