bug#2341: absolute file name arg of `true-filename': should/must?

[Juanma Barranquero <lekktu@gmail.com>]

Package: emacs,documentation

According to true-filename's docstring:

  Return the truename of filename, which should be absolute.

In (elisp)25.6.3 Truenames:

  The function `file-truename' returns the truename of the file
  FILENAME.  The argument must be an absolute file name.

OTOH,

  (file-truename "test") => "C:/emacs/test"

and some functions seem to use it as expecting that it expands the
argument. For example, `find-buffer-visiting':

  (defun find-buffer-visiting (filename &optional predicate)
    "..."
    (let ((predicate (or predicate #'identity))
          (truename (abbreviate-file-name (file-truename filename))))
       ;; ...

where the argument FILENAME of `find-buffer-visiting' is usually not
an absolute file name.

The current behavior seems to be: "SHOULD be an absolute filename; if
it is not, it will be expanded using `default-directory'." Is that
intended, and if so, shouldn't it be documented?

    Juanma