std::fmt: Added argument index comments to examples for specifying precision

The examples for specifying the precision have comments explaining which
argument the specifier is referring to. However, for implicit positional
arguments, the examples simply talk about "next arg". To make it easier for
readers to follow the comments, "next arg" was supplemented with the actual
resulting argument index.

Author: EliasHolzmann

library/alloc/src/fmt.rs

@@ -240,19 +240,19 @@
 //! // Hello {arg 0 ("x")} is {arg 2 (0.01) with precision specified in arg 1 (5)}
 //! println!("Hello {0} is {2:.1$}", "x", 5, 0.01);
 //!
-//! // Hello {next arg ("x")} is {second of next two args (0.01) with precision
-//! //                          specified in first of next two args (5)}
+//! // Hello {next arg -> arg 0 ("x")} is {second of next two args -> arg 2 (0.01) with precision
+//! //                          specified in first of next two args -> arg 1 (5)}
 //! println!("Hello {} is {:.*}",    "x", 5, 0.01);
 //!
 //! // Hello {arg 1 ("x")} is {arg 2 (0.01) with precision
-//! //                          specified in next arg (5)}
+//! //                          specified in next arg -> arg 0 (5)}
 //! println!("Hello {1} is {2:.*}",  5, "x", 0.01);
 //!
-//! // Hello {next arg ("x")} is {arg 2 (0.01) with precision
-//! //                          specified in next arg (5)}
+//! // Hello {next arg -> arg 0 ("x")} is {arg 2 (0.01) with precision
+//! //                          specified in next arg -> arg 1 (5)}
 //! println!("Hello {} is {2:.*}",   "x", 5, 0.01);
 //!
-//! // Hello {next arg ("x")} is {arg "number" (0.01) with precision specified
+//! // Hello {next arg -> arg 0 ("x")} is {arg "number" (0.01) with precision specified
 //! //                          in arg "prec" (5)}
 //! println!("Hello {} is {number:.prec$}", "x", prec = 5, number = 0.01);
 //! ```