Add code comments to destination packages

As part of the effort to improve readability of upload in workhorse
codebase, code comments were added to explain what each package is
designed for.

workhorse/internal/upload/destination/destination.go

+// The destination package handles uploading to a specific destination (delegates
+// to filestore or objectstore packages) based on options from the pre-authorization
+// API and finalizing the upload.
 package destination
 
 import (

workhorse/internal/upload/destination/filestore/filestore.go

+// The filestore package has a consumer specific to uploading to local disk storage.
 package filestore
 
 import (

workhorse/internal/upload/destination/objectstore/doc.go

+// The objectstore package consists of a number of consumers specific to uploading
+// to object storage providers that are S3 compatible (e.g. AWS S3, GCP, Azure).
+package objectstore