Add examples by @pnkfelix to fmt precision

Fixes #24656

Author: steveklabnik

src/libcollections/fmt.rs

@@ -407,6 +407,21 @@
 //! For floating-point types, this indicates how many digits after the decimal
 //! point should be printed.
 //!
+//! By example, these:
+//!
+//! ```
+//! println!("Hello {1} is {2:.0$}", 5, "x", 0.01);
+//! println!("Hello {0} is {2:.1$}", "x", 5, 0.01);
+//! println!("Hello {} is {:.*}",    "x", 5, 0.01);
+//! println!("Hello {} is {2:.*}",   "x", 5, 0.01);
+//! ```
+//!
+//! All print the same thing:
+//!
+//! ```text
+//! Hello x is 0.01000
+//! ```
+//!
 //! # Escaping
 //!
 //! The literal characters `{` and `}` may be included in a string by preceding