document all commands on the man pages

## I'm submitting a…

[ ] Bug

[x] Feature Request

[ ] Documentation Request

[ ] Other (Please describe in detail)

## Current Behavior

I looked at the i3 manpages hoping for a reference on the `for_window` command, and found only a `(see HTML userguide)` message.

## Desired Behavior

The manpages should provide complete documentation.

## Environment

Output of `i3 --moreversion 2>&-`:

Binary i3 version:  4.16.1 (2019-01-27) © 2009 Michael Stapelberg and contributors

Running i3 version: 4.16.1 (2019-01-27) (pid 2092)abort…)

Loaded i3 config: /home/doti/.config/i3/config (Last modified: Sun 27 Sep 2020 02:05:17 PM -03, 15 seconds ago)


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

The i3 binary you are running: i3

- Linux Distribution & Version: Devuan beowulf

- Are you using a compositor (e.g., xcompmgr or compton): No