Rollup merge of rust-lang#33129 - GuillaumeGomez:fmt_doc, r=steveklabnik

Doc improvement on std::fmt module

Part of rust-lang#29355.

r? @steveklabnik

Author: Manishearth

src/libcollections/fmt.rs

@@ -521,12 +521,24 @@ use string;
 ///
 /// # Examples
 ///
+/// Basic usage:
+///
 /// ```
 /// use std::fmt;
 ///
 /// let s = fmt::format(format_args!("Hello, {}!", "world"));
 /// assert_eq!(s, "Hello, world!".to_string());
 /// ```
+///
+/// Please note that using [`format!`][format!] might be preferrable.
+/// Example:
+///
+/// ```
+/// let s = format!("Hello, {}!", "world");
+/// assert_eq!(s, "Hello, world!".to_string());
+/// ```
+///
+/// [format!]: ../macro.format!.html
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn format(args: Arguments) -> string::String {
     let mut output = string::String::new();

src/libcore/fmt/mod.rs

@@ -776,6 +776,32 @@ pub trait UpperExp {
 ///
 ///   * output - the buffer to write output to
 ///   * args - the precompiled arguments generated by `format_args!`
+///
+/// # Examples
+///
+/// Basic usage:
+///
+/// ```
+/// use std::fmt;
+///
+/// let mut output = String::new();
+/// fmt::write(&mut output, format_args!("Hello {}!", "world"))
+///     .expect("Error occurred while trying to write in String");
+/// assert_eq!(output, "Hello world!");
+/// ```
+///
+/// Please note that using [`write!`][write_macro] might be preferrable. Example:
+///
+/// ```
+/// use std::fmt::Write;
+///
+/// let mut output = String::new();
+/// write!(&mut output, "Hello {}!", "world")
+///     .expect("Error occurred while trying to write in String");
+/// assert_eq!(output, "Hello world!");
+/// ```
+///
+/// [write_macro]: ../macro.write!.html
 #[stable(feature = "rust1", since = "1.0.0")]
 pub fn write(output: &mut Write, args: Arguments) -> Result {
     let mut formatter = Formatter {