Document -k option as experimental

strace -k does not produce a reliable output on all supported
configurations yet, even basic strace-k.test is known to fail
on some of them.

* strace.c (usage): Document -k option as experimental.
* strace.1: Likewise.
* NEWS: Likewise.

NEWS

@@ -6,7 +6,7 @@ Noteworthy changes in release 4.9 (????-??-??)
     added --enable-arm-oabi option to enable it at configure time.
 
 * Improvements
-  * Added -k option to print stack trace after each traced syscall.
+  * Added experimental -k option to print stack trace after each traced syscall.
   * Added -w option to produce stats on syscall latency.
     (addresses Debian bug #457497).
   * Added ARC architecture support.

strace.1

@@ -268,7 +268,7 @@ Print the help summary.
 Print the instruction pointer at the time of the system call.
 .TP
 .B \-k
-Print the execution stack trace of the traced processes after each system call.
+Print the execution stack trace of the traced processes after each system call (experimental).
 .TP
 .B \-q
 Suppress messages about attaching, detaching etc.  This happens

strace.c

@@ -238,7 +238,7 @@ usage: strace [-CdffhiqrtttTvVxxy] [-I n] [-e expr]...\n\
 -P path -- trace accesses to path\n\
 "
 #ifdef USE_LIBUNWIND
-"-k obtain stack trace between each syscall\n\
+"-k obtain stack trace between each syscall (experimental)\n\
 "
 #endif
 /* ancient, no one should use it