Update documentation for g:VimuxCommandShell

doc/vimux.txt

@@ -431,5 +431,21 @@ Caution: It is probably best not to mix this with |VimuxRunnerName|.
 <
 Default: 0
 
+------------------------------------------------------------------------------
+                                                            *VimuxCommandShell*
+4.12 g:VimuxCommandShell~
+
+Set this option to `1` or `v:true` to enable shell completion in
+VimuxPromptCommand
+Set this option to `0` or `v:false` to enable vim command editing in
+VimuxPromptCommand
+
+Enabling shell completion blocks the ability to use up-arrow to cycle through
+previously-run commands in VimuxPromptCommand.
+>
+  let g:VimuxCommandShell = 0
+<
+Default: 1
+
 ==============================================================================
 vim:tw=78:ts=2:sw=2:expandtab:ft=help:norl: