Reorganizes dynamic variables closes #2813

Adds `$*DEFAULT-READ-ELEMS` closes #2802 in the new section.

Author: JJ

doc/Language/variables.pod6

@@ -221,27 +221,27 @@ I<Note:> So far, if you use rakudo perl6, the example below cannot run
 correctly in the REPL. Please test it by copy-pasting it into a file, then
 run the file.
 
-    =begin code
-    my $lexical   = 1;
-    my $*dynamic1 = 10;
-    my $*dynamic2 = 100;
+=begin code
+my $lexical   = 1;
+my $*dynamic1 = 10;
+my $*dynamic2 = 100;
 
-    sub say-all() {
-        say "$lexical, $*dynamic1, $*dynamic2";
-    }
+sub say-all() {
+    say "$lexical, $*dynamic1, $*dynamic2";
+}
 
-    say-all();    # OUTPUT: 1, 10, 100
+say-all();    # OUTPUT: 1, 10, 100
 
-    {
-        my $lexical   = 2;
-        my $*dynamic1 = 11;
-        $*dynamic2    = 101;
+{
+    my $lexical   = 2;
+    my $*dynamic1 = 11;
+    $*dynamic2    = 101;
 
-        say-all(); # OUTPUT: 1, 11, 101
-    }
+    say-all(); # OUTPUT: 1, 11, 101
+}
 
-    say-all();  # OUTPUT: 1, 10, 101
-    =end code
+say-all();  # OUTPUT: 1, 10, 101
+=end code
 
 The first time C<&say-all> is called, it prints "C<1, 10, 100>" just as one
 would expect. The second time though, it prints "C<1, 11, 101>". This is
@@ -259,17 +259,17 @@ runtime L<failure|/type/Failure>, so a dynamic variable can be used
 undeclared as long as it's checked for definedness or used in a
 boolean context before using it for anything else:
 
-    =begin code
-    sub foo() {
-        $*FOO // 'foo';
-    }
+=begin code
+sub foo() {
+    $*FOO // 'foo';
+}
 
-    say foo; # OUTPUT: «foo␤»
+say foo; # OUTPUT: «foo␤»
 
-    my $*FOO = 'bar';
+my $*FOO = 'bar';
 
-    say foo; # OUTPUT: «bar␤»
-    =end code
+say foo; # OUTPUT: «bar␤»
+=end code
 
 Dynamic variables can have lexical scope when declared with C<my> or package
 scope when declared with C<our>. Dynamic resolution and resolution through
@@ -1120,9 +1120,10 @@ This I<magic> behavior of C<@()> has been deprecated as of 6.d
 C<$/> can have named attributes if the L<Regex|/language/regexes> had named
 capture-groups in it, or if the Regex called out to another Regex.
 
-    'I... see?' ~~ / \w+ $<punctuation>=[ <-[\w\s]>+ ] \s* $<final-word> = [ \w+ . ] /;
-    say $/<punctuation>; # OUTPUT: «｢....｣␤»
-    say $/<final-word>;  # OUTPUT: «｢see?｣␤»
+=for code
+'I... see?' ~~ / \w+ $<punctuation>=[ <-[\w\s]>+ ] \s* $<final-word> = [ \w+ . ] /;
+say $/<punctuation>; # OUTPUT: «｢....｣␤»
+say $/<final-word>;  # OUTPUT: «｢see?｣␤»
 
 These can also be accessed by the shortcut C«$<named>».
 
@@ -1512,13 +1513,6 @@ X<|$*TMPDIR>
 This is an L<IO::Path> object representing the "system temporary directory" as
 determined by L«C<.tmpdir IO::Spec::* method>|/routine/tmpdir».
 
-X<|$*TOLERANCE>
-=head4 C<$*TOLERANCE>
-
-Variable used by the L<C<=~=>|/routine/=~=> operator, and any operations that
-depend on it, to decide if two values are approximately equal. Defaults to
-C<1e-15>.
-
 X<|$*THREAD>
 =head4 C<$*THREAD>
 
@@ -1561,12 +1555,30 @@ sub MAIN($a, :$b, UInt :$ehehe) {
 
 It is accessible only inside of MAIN sub.
 
+=head3 Runtime variables
+
+These variables affect the behavior of certain functions, and in some cases its
+value can be changed during runtime.
+
 X<|$*COLLATION>
 =head4 C<$*COLLATION>
 
 This is a L<Collation|/type/Collation> object that can be used to
 configure Unicode collation levels.
 
+X<|$*TOLERANCE>
+=head4 C<$*TOLERANCE>
+
+Variable used by the L<C<=~=>|/routine/=~=> operator, and any operations that
+depend on it, to decide if two values are approximately equal. Defaults to
+C<1e-15>.
+
+X<|$*DEFAULT-READ-ELEMS>
+=head4 C<$*DEFAULT-READ-ELEMS>
+
+Affects the number of bytes read by default by
+L<C<IO::Handle.read>|/type/IO::Handle#method_read>. Its default value is 65536.
+
 =head1 Naming conventions
 
 It is helpful to know our naming conventions in order to understand what codes