Add documentation for scratchpad_state field present in the output of the command i3-msg -t get_tree

[slashfoo]

I'm submitting a…

[ ] Bug
[ ] Feature Request
[X] Documentation Request
[ ] Other (Please describe in detail)

Current Behavior

Browsing the TREE reply of the IPC interface (interprocess communication) there is no mention of the field scratchpad_state, this field is present in the output of running i3-msg -t get_tree.

Desired Behavior

The document should mention the meaning of the field scratchpad_state, it's possible values, and perhaps the conditions that cause a change in these values.

Impact

[ ] This feature requires new configuration and/or commands

Environment

Output of i3 --moreversion 2>&-:

Binary i3 version:  4.19.1-non-git © 2009 Michael Stapelberg and contributors
Running i3 version: 4.19.1-non-git (pid 2291)
Loaded i3 config: /home/jamielalmeida/.config/i3/config (Last modified: Mon 19 Jul 2021 07:29:28 PM PDT, 141549 seconds ago)

The i3 binary you just called: /usr/bin/i3
The i3 binary you are running: i3

- Linux Distribution & Version: Debian GNU/Linux 11
- Are you using a compositor (e.g., xcompmgr or compton): picom