bug#42296: 27.0.91; Correct manual entry for 'concat' w.r.t. allocation [PATCH]

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Sat, 11 Jul 2020 14:51:06 +0200
> Cc: 42296@debbugs.gnu.org
> 
> 11 juli 2020 kl. 13.52 skrev Eli Zaretskii <eliz@gnu.org>:
> 
> > Too definitive to my palate, sorry.
> 
> Apologies -- I do my best to read your mind.

I posted my proposed text so that you won't have to.

>  This function does not always allocate a new string.  Callers are
>  advised not rely on the result being a new string nor on it being @code{eq}
>  to an existing string.
> 
>  In particular, mutating the returned value may inadvertently change another
>  string, alter a constant string in the program, or even raise an error.
>  To obtain a string that can be mutated, use @code{copy-sequence} on the 
> result.

Fine with me, with one correction: last sentence will sound better if
reworded like this:

  To obtain a string that you can safely mutate, use
  @code{copy-sequence} on the result.