Clarify topic variable's status in for loop

Issue: #3075

Author: uzluisf

doc/Language/control.pod6

@@ -562,11 +562,19 @@ for $*IN.lines -> $line { .say }
 
 Iteration variables are always lexical, so you don't need to use C<my> to give
 them the appropriate scope. Also, they are read-only aliases. If you need them
-to be read-write, use C«<->» instead of C«->». If you need to make C<$_>
-read-write in a for loop, do so explicitly.
+to be read-write, use C«<->» instead of C«->».
 
     my @foo = 1..3;
-    for @foo <-> $_ { $_++ }
+    for @foo <-> $value { $value++ }
+
+By default the topic variable C«$_» is a read-write alias. Thus, if you want
+to use it as a read-only alias you must indicate it by using C«->».
+
+    my @foo = 1..3;
+    for @foo -> $_ { $_.say }
+
+    # Error: ...require mutable arguments
+    for @foo -> $_ { $_++ }
 
 A C<for> loop can produce a C<List> of the values produced by each run of the
 attached block. To capture these values, put the for loop in parenthesis or