Document method's intent a little better

This is in anticipation of cleaning up this method

lib/friendly_id/slugged.rb

@@ -252,10 +252,17 @@ def normalize_friendly_id(value)
     def should_generate_new_friendly_id?
       base       = send(friendly_id_config.base)
       slug_value = send(friendly_id_config.slug_column)
+
+      # If the slug base is nil, and the slug field is nil, then we're going to
+      # leave the slug column NULL.
       return false if base.nil? && slug_value.nil?
+      # Otherwise, if this is a new record, we're definitely going to try to
+      # create a new slug.
       return true if new_record?
       slug_base = normalize_friendly_id(base)
       separator = Regexp.escape friendly_id_config.sequence_separator
+      # If the slug base (without sequence) is different from either the current
+      # friendly id or the slug value, then we'll generate a new friendly_id.
       slug_base != (current_friendly_id || slug_value).try(:sub, /#{separator}[\d]*\z/, '')
     end