Better documentation

Protobuf.podspec

@@ -15,14 +15,14 @@ Pod::Spec.new do |s|
                    'objectivec/google/protobuf/SourceContext.pbobjc.{h,m}',
                    'objectivec/google/protobuf/Timestamp.pbobjc.h',
                    'objectivec/google/protobuf/Wrappers.pbobjc.{h,m}'
-  # The following would cause duplicate symbol definitions
+  # The following would cause duplicate symbol definitions. GPBProtocolBuffers is expected to be
+  # left out, as it's an umbrella implementation file. For Api, Struct and Type, see issue #449.
   s.exclude_files = 'objectivec/GPBProtocolBuffers.m',
                     'objectivec/google/protobuf/Api.pbobjc.{h,m}',
                     'objectivec/google/protobuf/Struct.pbobjc.{h,m}',
                     'objectivec/google/protobuf/Type.pbobjc.{h,m}'
-  # The .m's of the proto Well-Known-Types under google/protobuf are #imported
-  # by GPBWellKnownTypes.m. So we can't compile them (duplicate symbols), but we
-  # need them available for the importing:
+  # Timestamp.pbobjc.m and Duration.pbobjc.m are #imported by GPBWellKnownTypes.m. So we can't
+  # compile them (duplicate symbols), but we need them available for the importing:
   s.preserve_paths = 'objectivec/google/protobuf/Timestamp.pbobjc.m',
                      'objectivec/google/protobuf/Duration.pbobjc.m'
   s.header_mappings_dir = 'objectivec'