runtime(vimcomplete): do not complete 'shellcmd' on WSL and Windows

- shellcmd completion is VERY slow on both WSL and Windows, e.g. `term
  something` or `!something` might take ~10 seconds to show first
  results. Do not complete it there.

- revert previous change to not complete on whitespace, do not complete
  on *empty* lines instead.

closes: #18568

Signed-off-by: Maxim Kim <habamax@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>

runtime/doc/insert.txt

@@ -1,4 +1,4 @@
-*insert.txt*    For Vim version 9.1.  Last change: 2025 Oct 14
+*insert.txt*    For Vim version 9.1.  Last change: 2025 Oct 16
 
 
 		  VIM REFERENCE MANUAL    by Bram Moolenaar
@@ -1704,6 +1704,36 @@ Notes:
 <  to your vimrc
 
 
+VIM							*ft-vim-omni*
+
+Simple completion of Vimscript and Vim9script languages.
+
+Complete:
+
+- set and & options
+- commands and command arguments
+- function names after ->
+- expressions
+- l:, v:, g:, s: and b: variables
+- fallback to command line completion to get candidates
+
+Notes
+
+- It doesn't complete command arguments that rely on 'shellcmd' completion
+  type in Windows and WSL due to general slowness of canditate gathering,
+  e.g.
+>
+     terminal dir
+     !dir
+<
+  These completions might take several seconds to gather candidates.
+
+- 'autocomplete' can't complete "no" options:
+>
+     set noautoindent
+     set nobuflisted
+<
+
 SYNTAX							*ft-syntax-omni*
 
 Vim has the ability to color syntax highlight nearly 500 languages.  Part of