docs: add comment for OSC 11 tmux passthrough (#26566)

runtime/lua/vim/_defaults.lua

@@ -299,9 +299,15 @@ if tty then
     })
 
     local query = '\027]11;?\007'
+
+    -- tmux 3.3a and earlier do not query the parent terminal for background color. As of the
+    -- writing of this comment, 3.3a is the latest release, so a passthrough sequence is necessary.
+    -- The passthrough should be removed as soon as a tmux version later than 3.3a is released.
+    -- See: https://github.com/neovim/neovim/pull/26557
     if os.getenv('TMUX') then
       query = string.format('\027Ptmux;%s\027\\', query:gsub('\027', '\027\027'))
     end
+
     io.stdout:write(query)
 
     timer:start(1000, 0, function()