Add explanatory comment to upload_files

lib/phraseapp_updater/phraseapp_api.rb

@@ -97,15 +97,16 @@ def download_files(locales, skip_unverified:)
       end
     end
 
-    # Empirically, PhraseApp fails to parse the uploaded files when uploaded in
-    # parallel. Give it a better chance by uploading them one at a time.
     def upload_files(locale_files, default_locale:)
       locale_files = locale_files.sort_by(&:locale_name)
       default_locale_file = locale_files.detect { |l| l.locale_name == default_locale }
       locale_files.delete(default_locale_file) if default_locale_file
 
       known_locales = fetch_locales.index_by(&:name)
 
+      # Phraseapp appears to use to use the first file uploaded to resolve conflicts
+      # between pluralized and non-pluralized keys. Upload and verify the canonical
+      # default locale first before uploading translated locales.
       if default_locale_file
         unless known_locales.has_key?(default_locale_file.locale_name)
           STDERR.puts("Creating default locale (#{default_locale_file})")