Write some docs for ToString

Part of #29376

Author: steveklabnik

src/libcollections/string.rs

@@ -1179,10 +1179,28 @@ impl PartialEq for ParseError {
 #[stable(feature = "str_parse_error", since = "1.5.0")]
 impl Eq for ParseError {}
 
-/// A generic trait for converting a value to a string
+/// A trait for converting a value to a `String`.
+///
+/// This trait is automatically implemented for any type which implements the
+/// [`Display`] trait. As such, `ToString` shouldn't be implemented directly:
+/// [`Display`] should be implemented instead, and you get the `ToString`
+/// implementation for free.
+///
+/// [`Display`]: ../fmt/trait.Display.html
 #[stable(feature = "rust1", since = "1.0.0")]
 pub trait ToString {
-    /// Converts the value of `self` to an owned string
+    /// Converts the given value to a `String`.
+    ///
+    /// # Examples
+    ///
+    /// Basic usage:
+    ///
+    /// ```
+    /// let i = 5;
+    /// let five = String::from("5");
+    ///
+    /// assert_eq!(five, i.to_string());
+    /// ```
     #[stable(feature = "rust1", since = "1.0.0")]
     fn to_string(&self) -> String;
 }