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 13:15:39 +0200
> Cc: 42296@debbugs.gnu.org
> 
>  In particular, the returned value should not be altered, because doing
>  so may inadvertently change another string, alter a constant string in
>  the program, or even raise an error.

Too definitive to my palate, sorry.

You may wish reading the recent long discussions of what is/should be
and what isn't/shouldn't be immutable.  Turns out some of us are very
sensitive to "should not" being used where "we recommend you not" or
"make sure you know what you are doing" is what actually is the truth.

Once again, we are not talking to kindergarten kids.