Adds more env vars from the wiki by @MasterDuke17

This would close #949.

Author: JJ

doc/Programs/03-environment-variables.pod6

@@ -1,11 +1,16 @@
 =begin pod
 
-=TITLE Shell environment variables used by the interpreter
+=TITLE Shell environment variables used by Perl 6
 
 =SUBTITLE What variables you can declare to alter its behavior.
 
-Rakudo's behavior can be tweaked by a (growing) number of environment variables;
-this section attempts to document all those currently in use.
+The behavior of the interpreter can be tweaked by a (growing) number of
+environment variables; this section attempts to document all those currently in
+use. All these variables correspond to the L<Rakudo|https://rakudo.org>, and
+they are interpreter specific in all cases, except where some use conventional
+names such as C<PATH>.
+
+The underlying virtual machine is also sensitive to a series of environment variables, they are listed L<in this wiki page|https://github.com/rakudo/rakudo/wiki/dev-env-vars#moarvm>
 
 =head2 Module Loading
 
@@ -17,9 +22,15 @@ information to standard error.
 
 =head2 Error Message Verbosity and Strictness
 
+=item C<RAKUDO_EXCEPTIONS_HANDLER> If present, the print_exception routine will
+use a class of that name to process the exception for output. Rakudo currently
+ships with Exceptions::JSON (invoked by setting this variable to C<JSON>, to
+override the default output.
+
 =item C<RAKUDO_NO_DEPRECATIONS>
 
-If true, suppresses deprecation warnings triggered by the C<is DEPRECATED> trait.
+If true, suppresses deprecation warnings triggered by the C<is DEPRECATED>
+trait.
 
 =item C<RAKUDO_DEPRECATIONS_FATAL>
 
@@ -33,22 +44,36 @@ Displays source code in stack frames surrounded by the specified number of lines
 
 Controls whether .setting files are included in backtraces.
 
-=back
-
 =head2 Affecting Precompilation
 
 =item C<RAKUDO_PREFIX>
 
-When this is set, Rakudo will look for the standard repositories (perl, vendor, site) in the
-specified directory. This is intended as an escape hatch for build-time bootstrapping issues,
-where Rakudo may be built as an unprivileged user without write access to the runtime paths
-in NQP's config.
+When this is set, Rakudo will look for the standard repositories (perl, vendor,
+site) in the specified directory. This is intended as an escape hatch for
+build-time bootstrapping issues, where Rakudo may be built as an unprivileged
+user without write access to the runtime paths in NQP's config.
 
 =item C<RAKUDO_PRECOMP_DIST>, C<RAKUDO_PRECOMP_LOADING> C<RAKUDO_PRECOMP_WITH>: These are internal variables for passing serialized state to precompilation jobs in child processes. Please do not set them manually.
 
+=item C<RAKUDO_LOG_PRECOMP> If set to 1, diagnostic information about the precompilation process is emitted.
+
+=head2 Line editor
+
+=item C<RAKUDO_LINE_EDITOR> This specifies the preferred line editor to use;
+valid values are Readline, Linenoise, and none. A value of none is useful if you
+want to avoid the recommendation message upon REPL startup.
+
+=item C<RAKUDO_DISABLE_MULTILINE> Set to 1, will disable multiline input for the REPL
+
+=item C<RAKUDO_HIST> This specifies the location of the history file used by the
+line editor; the default is F<~/.perl6/rakudo-history>.
+
 
 =head2 Other
 
+=item C<RAKUDO_DEFAULT_READ_ELEMS> Defaults to 65536; the default number of
+characters to read on an IO::Handle.
+
 =item C<RAKUDO_ERROR_COLOR> Controls whether to emit ANSI codes for error
 highlighting. Defaults to true if unset, except on Win32.