Update comment on #write_if_empty

lib/gitlab/diff/highlight_cache.rb

@@ -15,6 +15,7 @@ module Gitlab
 
       # - Reads from cache
       # - Assigns DiffFile#highlighted_diff_lines for cached files
+      #
       def decorate(diff_file)
         if content = read_file(diff_file)
           diff_file.highlighted_diff_lines = content.map do |line|
@@ -23,6 +24,11 @@ module Gitlab
         end
       end
 
+      # For every file that isn't already contained in the redis hash, store the
+      #   result of #highlighted_diff_lines, then submit the uncached content
+      #   to #write_to_redis_hash to submit a single write. This avoids excessive
+      #   IO generated by N+1's (1 writing for each highlighted line or file).
+      #
       def write_if_empty
         diff_files = @diff_collection.diff_files
         cached_diff_files = read_cache