Document how to change mux SSH_AUTH_SOCK

[phromo]

What page or section of the docs have an issue?
https://wezfurlong.org/wezterm/config/lua/config/mux_enable_ssh_agent.html

Describe the issue

Background:
I typically set SSH_AUTH_SOCK from my fish shell (which wezterm starts), my basic profile env holds an inappropriate value. When wezterm starts it seems to read SSH_AUTH_SOCK from the env and use that for the mux servers (as listed by wezterm cli list-clients).

Desired change:
Since in my case, what wezterm has picked up is wrong, I would like to have clearer documentation on how to change it. I can list it yes, but how to change it?

Additional notes
The doc says this: "The auth sock will point to a symbolic link that will in turn be pointed to the authentication socket associated with the most recently active multiplexer client."

Maybe that statement is intended to tell me what I should do? However, I cannot convert that statement into what I should do :/

[phromo]

It could be that the only solution is to ensure my env has a correct value for SSH_AUTH_SOCK before wezterm starts. If so, maybe add that explicitly to the docs?