[glib] gmacros: Clarify G_GNUC_[PRINTF|SCANF] macro documentation

[Philip Withnall <pwithnall src gnome org>]

commit d8acf9b956e23143f6463d7a632699d4a7761f4e
Author: Philip Withnall <philip withnall collabora co uk>
Date:   Wed Nov 9 15:11:53 2016 +0000

    gmacros: Clarify G_GNUC_[PRINTF|SCANF] macro documentation
    
    Clarify that the arguments parameter can be zero if the function being
    annotated just accepts a string format argument, and no varargs for it
    (for example, if it takes a va_list of arguments instead).
    
    Add some links to the GCC documentation for the `format` attribute.

 glib/docs.c |   18 ++++++++++++------
 1 files changed, 12 insertions(+), 6 deletions(-)
---
diff --git a/glib/docs.c b/glib/docs.c
index 781869d..5ac7354 100644
--- a/glib/docs.c
+++ b/glib/docs.c
@@ -2237,8 +2237,9 @@
 /**
  * G_GNUC_PRINTF:
  * @format_idx: the index of the argument corresponding to the
- *     format string (The arguments are numbered from 1)
- * @arg_idx: the index of the first of the format arguments
+ *     format string (the arguments are numbered from 1)
+ * @arg_idx: the index of the first of the format arguments, or 0 if
+ *     there are no format arguments
  *
  * Expands to the GNU C format function attribute if the compiler is gcc.
  * This is used for declaring functions which take a variable number of
@@ -2248,7 +2249,9 @@
  * Place the attribute after the function declaration, just before the
  * semicolon.
  *
- * See the GNU C documentation for more details.
+ * See the
+ * [GNU C 
documentation](https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-Wformat-3288)
+ * for more details.
  *
  * |[<!-- language="C" -->
  * gint g_snprintf (gchar  *string,
@@ -2261,15 +2264,18 @@
 /**
  * G_GNUC_SCANF:
  * @format_idx: the index of the argument corresponding to
- *     the format string (The arguments are numbered from 1)
- * @arg_idx: the index of the first of the format arguments
+ *     the format string (the arguments are numbered from 1)
+ * @arg_idx: the index of the first of the format arguments, or 0 if
+ *     there are no format arguments
  *
  * Expands to the GNU C format function attribute if the compiler is gcc.
  * This is used for declaring functions which take a variable number of
  * arguments, with the same syntax as scanf(). It allows the compiler
  * to type-check the arguments passed to the function.
  *
- * See the GNU C documentation for details.
+ * See the
+ * [GNU C 
documentation](https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#index-Wformat-3288)
+ * for details.
  */
 
 /**