backport: docs: misc, window (#39722)

- formalize `window-number` similar to `tabpage-number`.
- reference it from docs.

test/functional/options/defaults_spec.lua

@@ -30,6 +30,11 @@ local is_os = t.is_os
 local testlog = 'Xtest-defaults-log'
 
 describe('startup defaults', function()
+  it("NVIM_NOTTYFAST=1 unsets 'ttyfast'", function()
+    clear { env = { NVIM_NOTTYFAST = '1' } }
+    eq(0, n.eval('&ttyfast'))
+  end)
+
   describe(':filetype', function()
     local function expect_filetype(expected)
       local screen = Screen.new(50, 4)

test/functional/terminal/tui_spec.lua

@@ -3326,7 +3326,7 @@ describe('TUI', function()
     end)
     local child_session = n.connect(child_server)
     local expected_msg =
-      "defaults.lua: Did not detect DSR response from terminal for 'background' detection. This results in a slower startup time. To disable this and other 'ttyfast' features during startup, set the environment variable NVIM_NOTTYFAST"
+      "E1568: Terminal did not respond to DSR request for 'background' color. Startup may be slower. :help 'ttyfast'"
     retry(nil, 4000, function()
       eq({ true, { mode = 'n', blocking = false } }, { child_session:request('nvim_get_mode') })
       if not is_os('win') then -- ConPTY provides DSR response on Windows?