Minor doc adjustments

src/lib.rs

@@ -461,10 +461,11 @@ pub trait Rng {
     /// 
     /// This is the only method which allows an RNG to report errors while
     /// generating random data; other methods either handle the error
-    /// internally or panic. This method is intended to allow error-handling
-    /// when seeding keys or PRNGs from external (true) RNGs.
+    /// internally or panic. This method is
+    /// the intended way to use external (true) RNGs, like `OsRng`. Its main
+    /// use-cases are to generate keys and to seed (infallible) PRNGs.
     /// 
-    /// Other than error handling, this is identical to [`fill`], and
+    /// Other than error handling, this method is identical to [`fill`], and
     /// has a default implementation simply wrapping [`fill`].
     /// 
     /// [`fill`]: trait.Rng.html#method.fill