bug#37538: [PATCH] Add docstring for `tags-complete-tags-table-file'.

[Eli Zaretskii <eliz@gnu.org>]

> Cc: larsi@gnus.org, 37538@debbugs.gnu.org
> From: Hong Xu <hong@topbug.net>
> Date: Fri, 11 Oct 2019 17:48:16 -0700
> 
> >> The description was quite long and I don't think it is justifiable to copy so much text over here to the docstring, plus there are additional reference in the referred manual section.
> > 
> > If you show me what long description you had in mind, I could try
> > saying that more concisely as appropriate for a doc string.
> > 
> 
> This snippet in Programmed Completion section:
> 
>     • A flag specifying the type of completion operation to perform; see
>       *note Basic Completion::, for the details of those operations.
>       This flag may be one of the following values.
> 
>       ‘nil’
>            This specifies a ‘try-completion’ operation.  The function
>            should return ‘nil’ if there are no matches; it should return
>            ‘t’ if the specified string is a unique and exact match; and
>            it should return the longest common prefix substring of all
>            matches otherwise.
> 
>       ‘t’
>            This specifies an ‘all-completions’ operation.  The function
>            should return a list of all possible completions of the
>            specified string.

How about the below:

  (defun tags-complete-tags-table-file (string predicate what)
    "Complete STRING from file names in the current tags table.
  PREDICATE, if non-nil is a function to filter possible matches:
  if it returns nil, the match is ignored.  If PREDICATE is nil,
  every possible match is acceptable.
  WHAT is a flag specifying the type of completion: t means `all-completions'
  operation, any other value means `try-completions' operation.

  This function serves as COLLECTION argument to `completing-read',
  see the Info node `(elisp) Programmed Completion' for more detailed
  description of the arguments."