Mention the extremely useful small_printf() function

It came in real handy while debugging an issue that strace 'fixed'.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

winsup/cygwin/DevDocs/how-to-debug-cygwin.txt

@@ -126,3 +126,9 @@ set CYGWIN_DEBUG=cat.exe:gdb.exe
    program will crash, probably in small_printf.  At that point, a 'bt'
    command should show you the offending call to strace_printf with the
    improper format string.
+
+9. Debug output without strace
+
+   If you cannot use gdb, or if the program behaves differently using strace
+   for whatever reason, you can still use the small_printf() function to
+   output debugging messages directly to stderr.