Adds support for appledoc in generated code.  (#1928)

Convert mapping of proto comments to appledoc format so they show up in Xcode and cocoadocs.

Fixes https://github.com/google/protobuf/issues/1866
diff --git a/objectivec/Tests/unittest_objc.proto b/objectivec/Tests/unittest_objc.proto
index 914945e..e5577fa 100644
--- a/objectivec/Tests/unittest_objc.proto
+++ b/objectivec/Tests/unittest_objc.proto
@@ -34,6 +34,15 @@
 
 package protobuf_unittest;
 
+// Used to check that Headerdocs and appledoc work correctly. If these comments
+// are not handled correctly, Xcode will fail to build the tests.
+message TestGeneratedComments {
+  // This is a string that could contain stuff like
+  // mime types as image/* or */plain. Maybe twitter usernames
+  // like @protobuf, @google or @something.
+  optional string string_field = 1;
+}
+
 // Using the messages in unittest.proto, setup for recursive cases for testing
 // extensions at various depths.
 extend TestAllExtensions {