Reword first paragraph

src/os.rs

@@ -15,9 +15,9 @@ use std::fmt;
 use rand_core::{RngCore, Error, impls};
 
 /// A random number generator that retrieves randomness straight from the
-/// operating system. This is the preferred external source of randomness for
-/// most applications. Commonly it is used to initialize a user-space RNG, which
-/// can then be used with much lower overhead.
+/// operating system. This is the preferred external source of entropy for most
+/// applications. Commonly it is used to initialize a user-space RNG, which can
+/// then be used to generate random values with much less overhead than `OsRng`.
 ///
 /// You may prefer to use [`EntropyRng`] instead of `OsRng`. Is is unlikely, but
 /// not entirely theoretical, for `OsRng` to fail. In such cases `EntropyRng`