commands: Make the --poll flag a duration

So you can do:

```
hugo server --poll 700ms
```

See #8720

docs/content/en/commands/hugo_server.md

@@ -59,7 +59,7 @@ hugo server [flags]
       --noHTTPCache            prevent HTTP caching
       --noTimes                don't sync modification time of files
       --path-warnings          print warnings on duplicate target paths etc.
-      --poll                   use a poll based approach to watch for file system changes
+      --poll string            set this to a poll interval, e.g --poll 700ms, to use a poll based approach to watch for file system changes
   -p, --port int               port on which the server will listen (default 1313)
       --print-mem              print memory usage to screen at intervals
       --renderToDisk           render to Destination path (default is render to memory & serve from there)