bug#5572: 23.1; Undocumented key :value-to-internal in customize system

[Teemu Likonen]

Some customize options use key :value-to-internal with customize type
"repeat". One example is variable message-ignored-news-headers in
lisp/gnus/message.el:


(defcustom message-ignored-news-headers
  "^NNTP-Posting-Host:\\|^Xref:\\|^[BGF]cc:\\|^Resent-Fcc: [...]
  "*Regexp of headers to be removed unconditionally before posting."
  :group 'message-news
  :group 'message-headers
  :link '(custom-manual "(message)Message Headers")
  :type '(repeat :value-to-internal (lambda (widget value)
                                      (custom-split-regexp-maybe value))
                 :match (lambda (widget value)
                          (or (stringp value)
                              (widget-editable-list-match widget value)))
                 regexp))


(I cut one long line and added ellipsis "[...]".)

:value-to-internal key doesn't seem to be documented anywhere so I
consider this a documentation bug. Please add a description for the key
- perhaps to the info node "(elisp) Type Keywords" - because it seems
very useful for Emacs extension writers.