Update function name and docs.

src/lib.rs

@@ -100,8 +100,8 @@ impl<L: LogLevel> Verbosity<L> {
     }
 
     /// Whether any verbosity flags (either `--verbose` or `--quiet`)
-    /// have been specified on the command line.
-    pub fn is_set(&self) -> bool {
+    /// are present on the command line.
+    pub fn is_present(&self) -> bool {
         self.verbose != 0 || self.quiet != 0
     }