text: Title: Document how user input to Trigger on works
  Body: ### AutoKey is a Xorg application and will not function in a Wayland session. Do you use Xorg (X11) or Wayland?
  
  Xorg
  
  ### Has this issue already been reported?
  
  - [X] I have searched through the existing issues.
  
  ### Is this a question rather than an issue?
  
  - [X] This is not a question.
  
  ### What type of issue is this?
  
  Documentation
  
  ### Choose one or more terms that describe this issue:
  
  - [X] autokey triggers
  - [ ] autokey-gtk
  - [ ] autokey-qt
  - [ ] beta
  - [ ] bug
  - [ ] critical
  - [ ] development
  - [X] documentation
  - [ ] enhancement
  - [ ] installation/configuration
  - [ ] phrase expansion
  - [ ] scripting
  - [ ] technical debt
  - [X] user interface
  
  ### Other terms that describe this issue if not provided above:
  
  Undocumented feature
  
  ### Which Linux distribution did you use?
  
  N/A
  
  ### Which AutoKey GUI did you use?
  
  Qt
  
  ### Which AutoKey version did you use?
  
  0.96.0-beta.10
  
  ### How did you install AutoKey?
  
  From AutoKey repo  debs
  
  ### Can you briefly describe the issue?
  
  In a comment to #406, @luziferius mentions that the Trigger on dropdown selection field accepts user input. AFAIK, this is not documented anywhere. It needs to be expored (in the code and in use) and documented.
  
  ### Can the issue be reproduced?
  
  Always
  
  ### What are the steps to reproduce the issue?
  
  Clck on `Set` Abbreviations on an action
  Click on the Trigger on field
  Type something and see what happens ...
  
  ### What should have happened?
  
  Didn't expect that typing here was allowed
  
  ### What actually happened?
  
  Typing is allowed, but there is no guidance as to what is allowed or how it can be used.
  
  ### Do you have screenshots?
  
  _No response_
  
  ### Can you provide the output of the AutoKey command?
  
  ```bash
  N/A
  ```
  
  
  ### Anything else?
  
  The source code  will have to be examined to see how user input to this field is processed and how this feature can be used.