Documents 'return from module'. (#976)

Fixes #974.

doc/site/modularity.markdown

@@ -306,5 +306,19 @@ This sounds super hairy, but that's because cyclic dependencies are hairy in
 general. The key point here is that Wren *can* handle them in the rare cases
 where you need them.
 
+## Exiting a module early
+
+Although the `return` statement is normally used to exit from a [method](classes.html#methods) or a [function](functions.html), it can also be used from a module's top-level code to exit the module. For example, if the script consists of a single module, this code would exit the module (and therefore the script) early:
+
+<pre class="snippet">
+for (i in 1..2) {
+  if (i == 2) return
+  System.print(i)  //>  prints 1 but not 2
+}
+System.print(3)    //>  not reached
+</pre>
+
+Although it is not invalid to return a value, there is no way to access that value and it is therefore simply discarded.
+
 <br><hr>
 <a href="error-handling.html">&larr; Error Handling</a>

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