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

[Eli Zaretskii]

> From: Mattias Engdegård <mattiase@acm.org>
> Date: Thu, 9 Jul 2020 17:54:10 +0200
> 
> 'concat' hasn't guaranteed the allocation of a new string since 1997 at least 
> (79d137ffe7d). Remove claims to the contrary from the manual.

This loses too much useful information, IMO.  Let's please say that
the result is sometimes ("frequently"?), but not always, a new string,
and that programs should not rely on that aspect.

Thanks.