patch 9.1.1160: Ctrl-Y does not work well with "preinsert" when completing items

patch 9.1.1160: Ctrl-Y does not work well with "preinsert" when completing items

Problem:  The 'preinsert' feature requires Ctrl-Y to confirm insertion,
          but Ctrl-Y only works when the popup menu (pum) is displayed.
          Without enforcing this dependency, it could lead to confusing
          behavior or non-functional features.

Solution: Modify ins_compl_has_preinsert() to check for both 'menu' and
          'menuone' flags when 'preinsert' is set. Update documentation
          to clarify this requirement. This avoids adding complex
          conditional behaviors. (glepnir)

fixes: #16728
closes: #16753

Signed-off-by: glepnir <glephunter@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>