Add &repl

Document the &repl sub, completing one item from the 2021.06 checklist
(issue #3907)

Author: codesections

doc/Type/independent-routines.pod6

@@ -135,6 +135,52 @@ by the final statement in the file when C<$check> is not C<True>.
 =for code
 EVALFILE "foo.raku";
 
+=head2 sub repl
+I<Note>: C<repl> was introduced in release 2021.06 of the Rakudo compiler.
+
+Defined as:
+
+    sub repl()
+
+Pauses execution and enters a REPL (read-eval-print loop) in the current
+context.  This REPL is exactly like the one created when you run C<raku>
+L<without any arguments|Programs/04-running-raku#DESCRIPTION> except that
+you can access/modify the program's current context (such as lexical
+variables).
+
+For example, if you run this code:
+
+=begin code
+my $name = "Alice";
+
+say "Hello, $name";
+
+repl();
+
+say "Goodbye, $name"
+=end code
+
+then you'll get the output C<Hello, Alice> and then enter a REPL session
+(I<before> any output with "goodbye" is printed).  Your REPL session could
+go as follows:
+
+
+    =begin code :lang<shell>
+    Type 'exit' to leave
+    [0] > $name
+    Alice
+    [1] > $name = "Bob"
+    Bob
+    [2] > exit
+    =end code
+
+After exiting the REPL session, Raku will resume running the program;
+during this run, any changes you made in the REPL session will still be
+in effect.  Thus, after the session above, you'd get the output
+C<Goodbye, Bob> rather than C<Goodbye, Alice> as you would have without
+the REPL session.
+
+
 =head2 sub mkdir
 
 Defined as: