format, format_args: Make xref to std::fmt much more prominent

That xref contains the actual documentation for what format! does.
It should be very prominent - particularly, more so than the other
links.

library/alloc/src/macros.rs

@@ -79,10 +79,12 @@ macro_rules! vec {
 ///
 /// The first argument `format!` receives is a format string. This must be a string
 /// literal. The power of the formatting string is in the `{}`s contained.
-///
 /// Additional parameters passed to `format!` replace the `{}`s within the
 /// formatting string in the order given unless named or positional parameters
-/// are used; see [`std::fmt`] for more information.
+/// are used.
+///
+/// See [the formatting syntax documentation in `std::fmt`](../std/fmt/index.html)
+/// for details.
 ///
 /// A common use for `format!` is concatenation and interpolation of strings.
 /// The same convention is used with [`print!`] and [`write!`] macros,
@@ -91,7 +93,6 @@ macro_rules! vec {
 /// To convert a single value to a string, use the [`to_string`] method. This
 /// will use the [`Display`] formatting trait.
 ///
-/// [`std::fmt`]: ../std/fmt/index.html
 /// [`print!`]: ../std/macro.print.html
 /// [`write!`]: core::write
 /// [`to_string`]: crate::string::ToString

library/core/src/macros/mod.rs

@@ -849,7 +849,8 @@ pub(crate) mod builtin {
     /// assert_eq!(display, debug);
     /// ```
     ///
-    /// For more information, see the documentation in [`std::fmt`].
+    /// See [the formatting documentation in `std::fmt`](../std/fmt/index.html)
+    /// for details of the macro argument syntax, and further information.
     ///
     /// [`Display`]: crate::fmt::Display
     /// [`Debug`]: crate::fmt::Debug