patronictl --help was showing ctl function's docstring (#2845)

`patronictl` is implemented using `click` module, and that module uses the functions' docstrings for creating a helper text.

As a consequence the docstring for `ctl` function was being shown to the user, which doesn't make sense.

This PR fixes that issue by adding a user-friendly description to be shown on `patronictl --help`. We use a `\f` to tell `click` when to stop capturing text to show in the helper.

Note that `patronictl` commands are implemented using `@ctl.command` decorator, and we always provide them with `help` argument. That said, none of the subcommands are affected by the aforementioned issue, only the entry point of the CLI.

References: PAT-201.

patroni/ctl.py

@@ -264,7 +264,9 @@ role_choice = click.Choice(['leader', 'primary', 'standby-leader', 'replica', 's
 @click.option('-k', '--insecure', is_flag=True, help='Allow connections to SSL sites without certs')
 @click.pass_context
 def ctl(ctx: click.Context, config_file: str, dcs_url: Optional[str], insecure: bool) -> None:
-    """Entry point of ``patronictl`` utility.
+    """Command-line interface for interacting with Patroni.
+    \f
+    Entry point of ``patronictl`` utility.
 
     Load the configuration file.