Problem: completion: preinserted text highlighed using ComplMatchIns
Solution: Use highlighting group PreInsert and update the documentation
(Girish Palya).
When "preinsert" is included in 'completeopt', only the PreInsert
highlight group should be applied, whether autocompletion is active or not.
Previously, ComplMatchIns was used when autocompletion was not enabled.
Related to https://github.com/vim/vim/pull/18213.
closes: vim/vim#182542525c56e42
Co-authored-by: Girish Palya <girishji@gmail.com>