bug#50811: 28.0.50; Misleading Docstring for read-string function

[Codruț Constantin Gușoi]

While trying to solve and issue in an emacs pacage: 
https://github.com/Alexander-Miller/cfrs/pull/6 I encountered the following 
docstring entry for the read-string function:


```

If non-nil, second arg INITIAL-INPUT is a string to insert before reading.
  This argument has been superseded by DEFAULT-VALUE and should normally be nil
  in new code.  It behaves as INITIAL-CONTENTS in `read-from-minibuffer' (which
  see).
```


This is misleading since INITIAL-INPUT behaves differently from DEFAULT-VALUE:


```

;; No pre-completed value, returns "" when empty.
(read-string "PROMPT ")

;; Pre-completed with "A", returns "" when empty.
(read-string "PROMPT " "A")

;; Pre-completed with "A", returns "B" when empty.
(read-string "PROMPT " "A" nil "B")

;; No pre-completed value, returns "B" when empty.
(read-string "PROMPT " nil nil "B")

```


Not sure what should be done here, either remove that note, since both 
arguments do a different job, or change the code to pass around DEFAULT-VALUE 
instead of INITIAL-INPUT internally when INITIAL-INPUT is nil, as the docstring 
suggests. But the latter feels weird to me as getting rid of INITIAL-INPUT 
would be a breaking change so it's probably never going to be removed from the 
arguments list. Also I am not sure what the initial intention of superseding 
INITIAL-INPUT was.


Let me know what you think works best and I can submit patches.

Cheers,
Codruț

www.codrut.pro