with a suffix that is the section name to which it belongs.
If the value is `group', split completion candidates into groups
according to the sections.
+Since the values `annotation' and `group' rely on text properties,
+you can use them only by selecting candidates from the completions
+buffer, not by typing in the minibuffer.
Any other value is treated as `prefix'.
The value of `imenu-level-separator', a string, is used to separate