Mention time units explicitly in CLI interface

The force-poll flag does not mention time units as opposed to the
debounce flag and to make it even more confusing, the comment in the
code says something else than the output:
*** Polling for changes every 2000 ms

This patch makes it clear what units are used.

src/cli.rs

@@ -71,7 +71,7 @@ pub struct Args {
     /// Force using the polling backend.
     #[builder(default)]
     pub poll: bool,
-    /// Interval for polling. (seconds)
+    /// Interval for polling. (milliseconds)
     #[builder(default = "2")]
     pub poll_interval: u32,
     #[builder(default)]
@@ -200,7 +200,7 @@ where
                  .short("p")
                  .long("postpone"))
         .arg(Arg::with_name("poll")
-                 .help("Force polling mode")
+                 .help("Force polling mode (interval in milliseconds)")
                  .long("force-poll")
                  .value_name("interval"))
         .arg(Arg::with_name("no-shell")