patch 9.0.0346: :horizontal modifier not fully supported

Problem:    :horizontal modifier not fully supported.
Solution:   Also use :horizontal for completion and user commands.
            (closes #11025)
diff --git a/runtime/doc/map.txt b/runtime/doc/map.txt
index 69557d6..6beae6e 100644
--- a/runtime/doc/map.txt
+++ b/runtime/doc/map.txt
@@ -1724,11 +1724,11 @@
 					*<mods>* *<q-mods>* *:command-modifiers*
 	<mods>  The command modifiers, if specified. Otherwise, expands to
 		nothing. Supported modifiers are |:aboveleft|, |:belowright|,
-		|:botright|, |:browse|, |:confirm|, |:hide|, |:keepalt|,
-		|:keepjumps|, |:keepmarks|, |:keeppatterns|, |:leftabove|,
-		|:lockmarks|, |:noautocmd|, |:noswapfile| |:rightbelow|,
-		|:sandbox|, |:silent|, |:tab|, |:topleft|, |:unsilent|,
-		|:verbose|, and |:vertical|.
+		|:botright|, |:browse|, |:confirm|, |:hide|, |:horizontal|,
+		|:keepalt|, |:keepjumps|, |:keepmarks|, |:keeppatterns|,
+		|:leftabove|, |:lockmarks|, |:noautocmd|, |:noswapfile|
+		|:rightbelow|, |:sandbox|, |:silent|, |:tab|, |:topleft|,
+		|:unsilent|, |:verbose|, and |:vertical|.
 		Note that |:filter| is not supported.
 		Examples: >
 		    command! -nargs=+ -complete=file MyEdit