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

[Eli Zaretskii]

> From: Hong Xu <hong@topbug.net>
> Date: Sat, 28 Sep 2019 00:59:21 -0700
> 
> * lisp/progmodes/etags.el (tags-complete-tags-table-file): Add docstring.

Thanks.  A comment below.

> -(defun tags-complete-tags-table-file (string predicate what) ; Doc string?
> +(defun tags-complete-tags-table-file (string predicate what)
> +  "Complete file names in the current tags table.
> +The meaning of the arguments are the same as the function form of
> +COLLECTION as explained in Info node `(elisp) Programmed
> +Completion'."

The first line of the doc string should mention the arguments.  And we
should explain STRING and WHAT there, because those aren't passed to
all-completions and try-completion.