Document support for INSIDE_EMACS environment variable

Author: lizmat

doc/Programs/03-environment-variables.rakudoc

@@ -131,6 +131,14 @@ X<|Programs,RAKUDO_ERROR_COLOR>
 Controls whether to emit ANSI codes for error highlighting. Defaults to true
 if unset, except on Windows.
 
+X<|Programs,INSIDE_EMACS>
+=item C<INSIDE_EMACS>
+
+Supported as of release 2022.04 of the Rakudo compiler.  If specified with
+a true value, will B<not> try to load any of the line editing modules in
+the C<REPL>.  This allows the C<REPL> to be better integrated in the Emacs
+environment (which sets this environment variable).
+
 X<|Programs,RAKUDO_MAX_THREADS>
 =item C<RAKUDO_MAX_THREADS> (I<UInt>; B<src/core.c/ThreadPoolScheduler.pm6>)