runtime(doc): Improve command-line completion docs (vim/vim#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
2bbd0d30ee
Co-authored-by: Yee Cheng Chin <ychin.git@gmail.com>