Rollup merge of #30591 - SimonSapin:patch-15, r=aturon

The first line (paragraph?) of a doc-comment is what rustdoc shows when listing items of a module.

What makes `Instant` and `SystemTime` different is important enough to be there. (Though feel free to bikeshed the wording.)

src/libstd/time/mod.rs

@@ -23,6 +23,7 @@ pub use self::duration::Duration;
 mod duration;
 
 /// A measurement of a monotonically increasing clock.
+///  Opaque and useful only with `Duration`.
 ///
 /// Instants are always guaranteed to be greater than any previously measured
 /// instant when created, and are often useful for tasks such as measuring
@@ -42,8 +43,8 @@ mod duration;
 #[unstable(feature = "time2", reason = "recently added", issue = "29866")]
 pub struct Instant(time::Instant);
 
-/// A measurement of the system clock appropriate for timestamps such as those
-/// on files on the filesystem.
+/// A measurement of the system clock, useful for talking to
+/// external entities like the file system or other processes.
 ///
 /// Distinct from the `Instant` type, this time measurement **is not
 /// monotonic**. This means that you can save a file to the file system, then