Documents 'return from module'. (#976)

Fixes #974.

test/language/module/returns/module_return.wren

@@ -0,0 +1,4 @@
+// nontest
+System.print("foo")
+return
+System.print("bar")

test/language/module/returns/module_return_value.wren

@@ -0,0 +1,4 @@
+// nontest
+System.print("foo")
+return 42
+System.print("bar")

test/language/module/returns/return.wren

@@ -0,0 +1,3 @@
+System.print("foo") // expect: foo
+return
+System.print("bar")

test/language/module/returns/return_from_import.wren

@@ -0,0 +1,5 @@
+import "./module_return"
+
+System.print("baz") 
+// expect: foo
+// expect: baz

test/language/module/returns/return_value.wren

@@ -0,0 +1,3 @@
+System.print("foo") // expect: foo
+return 42
+System.print("bar")

test/language/module/returns/return_value_from_import.wren

@@ -0,0 +1,5 @@
+import "./module_return_value"
+
+System.print("baz") 
+// expect: foo
+// expect: baz