(completion-try-completion, completion-all-completions):

(completion-try-completion, completion-all-completions):
Remove ill-defined (and mistakenly installed and luckily never used nor
documented) `completion-styles' property.
(completion-initials-expand, completion-initials-all-completions)
(completion-initials-try-completion): New functions.
(completion-styles-alist): Add doc to each entry.
Add new `initials' entry.