feat(cmdwin): drop CHECK_CMDWIN

Allow window navigation/creation even while cmdwin is alive! 😱

runtime/doc/cmdline.txt

@@ -1223,9 +1223,12 @@ but it's not possible to open another cmdwin from there.  There is no
 nesting.
 
 								*E11*
-|CTRL-W| navigation/split commands and some operations (e.g. |:terminal|) emit
-E11. But mouse clicks may focus other windows. The cmdwin buffer is pinned by
-'winfixbuf', so the window cannot switch to a different buffer.
+|CTRL-W| commands work as usual: you can navigate between, split, resize, and
+close windows while the command-line window is open.  E11 is still emitted by
+operations that would leave or repurpose the cmdwin: opening a new tab page
+(CTRL-W_T, |:tabnew|) or turning its buffer into a terminal (|:terminal|).
+The cmdwin buffer is pinned by 'winfixbuf', so its window cannot switch to a
+different buffer.
 
 
 CLOSE

runtime/doc/news.txt

@@ -188,11 +188,11 @@ EDITOR
   kind/menu/info/abbr for the popup menu.
 • |cmdwin| (|q:|, |q/|, |q?|, |c_CTRL-F|) is implemented as a "normal"
   buffer+window instead of a nested-state modal loop:
+  • You can create (and navigate) windows while cmdwin is active. Chaos!
   • 'inccommand' works in cmdwin!
   • API calls (e.g. |nvim_buf_delete()|) that previously failed with
     "E11: Invalid in command-line window" while cmdwin was open, now work
     normally. Async plugins no longer need special |CmdwinLeave| workarounds.
-  • Removed most cmdwin-specific |E11| guards, except for window navigation.
   • The |cmdwin-char| is shown via 'statuscolumn'.
 • |gf| and |<cfile>| support `file://…` URIs.
 • |:log| opens log files.