Cygwin: devdoc: Mention the extremely useful small_printf() function

This function came in real handy many, many times over the year whenever
I needed to debug any issues with the MSYS2/Cygwin runtime, first
instance was while debugging an issue that strace 'fixed'.

However, this function was not mentioned anywhere I looked, so it took
me a good while to find out about it. Let's improve on that situation by
mentioning it explicitly in the documentation about debugging the
runtime.

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

Author: dscho

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

@@ -126,3 +126,14 @@ 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.
+
+   This function accepts slightly different format placeholders than the
+   POSIX printf() function you're used to, for example `%W` instead of `%ls`
+   to print UTF-16 strings (provided via `wchar_t *` pointers). For full
+   details, see the first comment in `winsup/cygwin/smallprint.cc`.