Rollup merge of #41720 - frewsxcv:duration-doc-examples, r=steveklabnik

Improvements to `std::time::Duration` doc examples.

Opened primarily for the last commit, in response to comments in #39949.

Author: frewsxcv

src/libstd/time/duration.rs

@@ -84,7 +84,10 @@ impl Duration {
     /// ```
     /// use std::time::Duration;
     ///
-    /// let five_seconds = Duration::from_secs(5);
+    /// let duration = Duration::from_secs(5);
+    ///
+    /// assert_eq!(5, duration.as_secs());
+    /// assert_eq!(0, duration.subsec_nanos());
     /// ```
     #[stable(feature = "duration", since = "1.3.0")]
     #[inline]
@@ -99,7 +102,10 @@ impl Duration {
     /// ```
     /// use std::time::Duration;
     ///
-    /// let five_seconds = Duration::from_millis(5000);
+    /// let duration = Duration::from_millis(2569);
+    ///
+    /// assert_eq!(2, duration.as_secs());
+    /// assert_eq!(569000000, duration.subsec_nanos());
     /// ```
     #[stable(feature = "duration", since = "1.3.0")]
     #[inline]
@@ -119,9 +125,24 @@ impl Duration {
     /// ```
     /// use std::time::Duration;
     ///
-    /// let five_seconds = Duration::new(5, 0);
-    /// assert_eq!(five_seconds.as_secs(), 5);
+    /// let duration = Duration::new(5, 730023852);
+    /// assert_eq!(duration.as_secs(), 5);
+    /// ```
+    ///
+    /// To determine the total number of seconds represented by the `Duration`,
+    /// use `as_secs` in combination with [`subsec_nanos`]:
+    ///
     /// ```
+    /// use std::time::Duration;
+    ///
+    /// let duration = Duration::new(5, 730023852);
+    ///
+    /// assert_eq!(5.730023852,
+    ///            duration.as_secs() as f64
+    ///            + duration.subsec_nanos() as f64 * 1e-9);
+    /// ```
+    ///
+    /// [`subsec_nanos`]: #method.subsec_nanos
     #[stable(feature = "duration", since = "1.3.0")]
     #[inline]
     pub fn as_secs(&self) -> u64 { self.secs }