Changes wrong description

This closes 3075

Author: JJ

doc/Language/5to6-nutshell.pod6

@@ -872,12 +872,13 @@ for my $car (@cars)  {...} # Perl 5; read-write
 for @cars  -> $car   {...} # Raku; read-only
 for @cars <-> $car   {...} # Raku; read-write
 
-If the default topic C<$_> is being used, it is also read-write.
+In Raku and unlike Perl, the default topic C<$_> will behave in the same way,
+becoming read-only when used as a topic variable.
 
 =for code :lang<perl5>
 for (@cars)      {...} # Perl 5; $_ is read-write
 =for code :preamble<no strict;>
-for @cars        {...} # Raku; $_ is read-write
+for @cars        {...} # Raku; $_ is read-only
 for @cars <-> $_ {...} # Raku; $_ is also read-write
 
 It is possible to consume more than one element of the list in each iteration

doc/Language/control.pod6

@@ -579,8 +579,8 @@ for @foo <-> $value {
 say @bar; # # OUTPUT: «[Odd Even Odd]»
 =end code
 
-By default the topic variable C«$_» is a read-write alias. Thus, if you want
-to use it as a read-only alias you must indicate it by using C«->».
+This rule also applies to the topic variable C«$_», which by default is a
+read-write alias; it will become read-only if it's used in a C«->» loop.
 
     my @foo = 1..3;
     for @foo -> $_ { $_.say }