reflect/protodesc: improve documentation

Change-Id: Id1c0e0e702a7b0eb011c71de23cd5be76345d741
Reviewed-on: https://go-review.googlesource.com/c/protobuf/+/220341
Reviewed-by: Damien Neil <dneil@google.com>
diff --git a/reflect/protodesc/desc.go b/reflect/protodesc/desc.go
index de9e82a..658cad4 100644
--- a/reflect/protodesc/desc.go
+++ b/reflect/protodesc/desc.go
@@ -2,8 +2,14 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 
-// Package protodesc provides for converting descriptorpb.FileDescriptorProto
-// to/from the reflective protoreflect.FileDescriptor.
+// Package protodesc provides functionality for converting
+// FileDescriptorProto messages to/from protoreflect.FileDescriptor values.
+//
+// The google.protobuf.FileDescriptorProto is a protobuf message that describes
+// the type information for a .proto file in a form that is easily serializable.
+// The protoreflect.FileDescriptor is a more structured representation of
+// the FileDescriptorProto message where references and remote dependencies
+// can be directly followed.
 package protodesc
 
 import (