runtime(doc): Improve command-line completion docs (#13331)

* Improve command-line completion docs

Add more details about 'ignorecase' and its effect on cmdline
completion.

Make sure keys used in wildmenu are properly documented and linked in the
keys' documentation entries, and in `:h index` for proper
cross-referencing, as wildmenu popup is slightly different from
insert-mode popup menu.

* Fix docs typos

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/doc/index.txt b/runtime/doc/index.txt
index a302c59..e7e99e2 100644
--- a/runtime/doc/index.txt
+++ b/runtime/doc/index.txt
@@ -1112,6 +1112,22 @@
 |c_<Insert>|	<Insert>	toggle insert/overstrike mode
 |c_<LeftMouse>|	<LeftMouse>	cursor at mouse click
 
+commands in wildmenu mode (see 'wildmenu')
+
+		<Up>		move up to parent / select the previous match
+		<Down>		move down to submenu / select the next match
+		<Left>		select the previous match / move up to parent
+		<Down>		select the next match / move down to submenu
+		<CR>		move into submenu when doing menu completion
+		other		stop completion and insert the typed character
+
+commands in wildmenu mode with 'wildoptions' set to "pum"
+
+		CTRL-E		stop completion and go back to original text
+		CTRL-Y		accept selected match and stop completion
+		<PageUp>	select a match several entries back
+		<PageDown>	select a match several entries forward
+
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 5. Terminal-Job mode				*terminal-job-index*