add additional documentation on the freeze command

Author: will-ockmore

docs/cli.md

@@ -130,13 +130,19 @@ my_window
 ## Freeze sessions
 
 ```
+tmuxp freeze
 tmuxp freeze <session_name>
+tmuxp freeze --force <session_name>
 ```
 
 You can save the state of your tmux session by freezing it.
 
 Tmuxp will offer to save your session state to `.json` or `.yaml`.
 
+If no session is specified, it will default to the attached session.
+
+If the `--force` argument is passed, it will overwrite any existing config file with the same name.
+
 (cli-load)=
 
 ## Load session