[PATCH] Document that -k is only available with libunwind

[Giedrius Statkevičius]

Without this the user may get confused why `man strace` reports that
there is a option called -k but it doesn't work.

* strace.1: Add that -k is only available when strace is compiled with
  libunwind support.
---
 strace.1 | 1 +
 1 file changed, 1 insertion(+)

diff --git a/strace.1 b/strace.1
index 1e69217..8c25322 100644
--- a/strace.1
+++ b/strace.1
@@ -269,6 +269,7 @@ 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 (experimental).
+Only available if compiled with libunwind support.
 .TP
 .B \-q
 Suppress messages about attaching, detaching etc.  This happens
-- 
2.3.1