mention canonicalize flag

Author: nhuurre

src/frontend/Typechecker.ml

@@ -778,7 +778,9 @@ let check_lvalue cf tenv = function
         add_warning loc
           "Nested multi-indexing on the left hand side of assignment does not \
            behave the same as nested indexing in expressions. This is \
-           considered a bug and will be disallowed in Stan 2.32.0." ;
+           considered a bug and will be disallowed in Stan 2.32.0. The \
+           indexing can be automatically fixed using the canonicalize flag for \
+           stanc." ;
         let lvalue_rvalue_types_differ =
           try
             let flat_type =

test/integration/cli-args/canonicalize/canonical.expected

@@ -238,7 +238,8 @@ generated quantities {
 Warning in 'deprecated.stan', line 36, column 2: Nested multi-indexing on the
     left hand side of assignment does not behave the same as nested indexing
     in expressions. This is considered a bug and will be disallowed in Stan
-    2.32.0.
+    2.32.0. The indexing can be automatically fixed using the canonicalize
+    flag for stanc.
   $ ../../../../../install/default/bin/stanc --print-canonical funs.stanfunctions
 // comment test comment
 void test(int x, int y) {

test/integration/cli-args/canonicalize/deprecations-only.expected

@@ -218,7 +218,8 @@ generated quantities {
 Warning in 'deprecated.stan', line 36, column 2: Nested multi-indexing on the
     left hand side of assignment does not behave the same as nested indexing
     in expressions. This is considered a bug and will be disallowed in Stan
-    2.32.0.
+    2.32.0. The indexing can be automatically fixed using the canonicalize
+    flag for stanc.
   $ ../../../../../install/default/bin/stanc --auto-format --canonicalize deprecations funs.stanfunctions
 // comment test comment
 void test(int x, int y) {

test/integration/good/warning/pretty.expected

@@ -2379,15 +2379,18 @@ transformed data {
 Warning in 'lvalue_multiindex.stan', line 6, column 3: Nested multi-indexing
     on the left hand side of assignment does not behave the same as nested
     indexing in expressions. This is considered a bug and will be disallowed
-    in Stan 2.32.0.
+    in Stan 2.32.0. The indexing can be automatically fixed using the
+    canonicalize flag for stanc.
 Warning in 'lvalue_multiindex.stan', line 7, column 3: Nested multi-indexing
     on the left hand side of assignment does not behave the same as nested
     indexing in expressions. This is considered a bug and will be disallowed
-    in Stan 2.32.0.
+    in Stan 2.32.0. The indexing can be automatically fixed using the
+    canonicalize flag for stanc.
 Warning in 'lvalue_multiindex.stan', line 9, column 3: Nested multi-indexing
     on the left hand side of assignment does not behave the same as nested
     indexing in expressions. This is considered a bug and will be disallowed
-    in Stan 2.32.0.
+    in Stan 2.32.0. The indexing can be automatically fixed using the
+    canonicalize flag for stanc.
   $ ../../../../../install/default/bin/stanc --auto-format matrix_pow_warning.stan
 data {
   int<lower=0> N;