Correcting the dmenu cmd in the docs (gopasspw#441)

* Correcting dmenu call

Since if the setting `safecontent` is set to `true`, it won't work like this. In order to be sure to get the password on the first line no matter what, you need to use the `-f` flag. (Which do nothing if `safecontent` is set to `false`)

* Adding some details

So it's all explained now.

docs/setup.md

@@ -189,15 +189,17 @@ Since writing fish completion scripts is not yet supported by the CLI library we
 
 In earlier versions gopass supported [dmenu](http://tools.suckless.org/dmenu/). We removed this and encourage you to call dmenu yourself now.
 
-This also makes it easier to call gopass with [rofi](https://github.com/DaveDavenport/rofi), for example.
+This also makes it easier to call gopass with any drop-in replacement of dmenu, like [rofi](https://github.com/DaveDavenport/rofi), for example, since you would just need to replace the `dmenu` call below by `rofi -dmenu`.
 
 ```bash
 # Simply copy the selected password to the clipboard
 gopass ls --flat | dmenu | xargs --no-run-if-empty gopass show -c
 # First pipe the selected name to gopass, encrypt it and type the password with xdotool.
-gopass ls --flat | dmenu | xargs --no-run-if-empty gopass show | head -n 1 | xdotool type --clearmodifiers --file -
+gopass ls --flat | dmenu | xargs --no-run-if-empty gopass show -f | head -n 1 | xdotool type --clearmodifiers --file -
 ```
 
+You can then simply bind these command lines to your preferred shortcuts in your window manager settings, typically under `System Settings > Keyboard > Shortcuts > Custom Shortcuts`.
+
 ### Filling in passwords from browser
 
 Gopass allows filling in passwords in browsers leveraging a browser plugin like [gopass bridge](https://github.com/martinhoefling/gopassbridge).