docs: Discourage use of gstdio.h and clarify its header requirements

Signed-off-by: Philip Withnall <withnall@endlessm.com>

https://bugzilla.gnome.org/show_bug.cgi?id=781598

glib/gfileutils.c

@@ -62,6 +62,10 @@
  * @title: File Utilities
  * @short_description: various file-related functions
  *
+ * Do not use these APIs unless you are porting a POSIX application to Windows.
+ * A more high-level file access API is provided as GIO — see the documentation
+ * for #GFile.
+ *
  * There is a group of functions which wrap the common POSIX functions
  * dealing with filenames (g_open(), g_rename(), g_mkdir(), g_stat(),
  * g_unlink(), g_remove(), g_fopen(), g_freopen()). The point of these
@@ -69,6 +73,11 @@
  * characters in them on Windows without having to use ifdefs and the
  * wide character API in the application code.
  *
+ * On some Unix systems, these APIs may be defined as identical to their POSIX
+ * counterparts. For this reason, you must check for and include the necessary
+ * header files (such as `fcntl.h`) before using functions like g_creat(). You
+ * must also define the relevant feature test macros.
+ *
  * The pathname argument should be in the GLib file name encoding.
  * On POSIX this is the actual on-disk encoding which might correspond
  * to the locale settings of the process (or the `G_FILENAME_ENCODING`