Document --repl-mode cmd opt

Impl: rakudo/rakudo@93e599dbf2
      Raku/nqp@943f7f757c

Author: zoffixznet

doc/Programs/00-running.pod6

@@ -11,9 +11,9 @@
 
 =head1 DESCRIPTION
 
-With no arguments, enters a REPL. With a C<[programfile]> or the C<-e>
-option, compiles the given program and, by default, also executes the
-compiled code.
+With no arguments, enters a REPL (see --repl-mode option).
+With a "[programfile]" or the "-e" option, compiles the given program
+and, by default, also executes the compiled code.
 
 =begin code :skip-test
 
@@ -46,6 +46,15 @@ compiled code.
                             any other extension outputs in HTML
   --doc=module         use Pod::To::[module] to render inline documentation
 
+  --repl-mode=interactive|non-interactive
+                       when running without "-e" or filename arguments,
+                       a REPL is started. By default, if STDIN is a TTY,
+                       "interactive" REPL is started that shows extra messages and
+                       prompts, otherwise a "non-interactive" mode is used where
+                       STDIN is read entirely and evaluated as if it were a program,
+                       without any extra output (in fact, no REPL machinery is even
+                       loaded). This option allows to bypass TTY detection and
+                       force one of the REPL modes.
 =end code
 
 Note that only boolean single-letter options may be bundled.