doc: clarify that pane ids are unique across sessions

[blueyed]

I was about proposing to unset g:tbone_write_pane after :Tattach, thinking it would use pane numbers there initially.

[tpope]

I find this wording confusing. Would changing "unchanging" to "globally unique" do the trick?

[blueyed]

Yes, better already.. but I am using -L with tmux and there is also -S.