List of available completion styles.
Each element has the form (NAME TRY-COMPLETION ALL-COMPLETIONS DOC):
where NAME is the name that should be used in `completion-styles',
TRY-COMPLETION is the function that does the completion (it should
follow the same calling convention as `completion-try-completion'),
ALL-COMPLETIONS is the function that lists the completions (it should
follow the calling convention of `completion-all-completions'),
and DOC describes the way this style of completion works.