comment some htmlifciation regexes

src/com/fsck/k9/helper/HtmlConverter.java

@@ -204,8 +204,14 @@ public class HtmlConverter {
             Log.e(K9.LOG_TAG, "Could not read string to convert text to HTML:", e);
         }
         text = buff.toString();
+
+        // Replace lines of -,= or _ with horizontal rules
         text = text.replaceAll("\\s*([-=_]{30,}+)\\s*", "<hr />");
+
+        // TODO: reverse engineer (or troll history) and document
         text = text.replaceAll("(?m)^([^\r\n]{4,}[\\s\\w,:;+/])(?:\r\n|\n|\r)(?=[a-z]\\S{0,10}[\\s\\n\\r])", "$1 ");
+
+        // Compress four or more newlines down to two newlines
         text = text.replaceAll("(?m)(\r\n|\n|\r){4,}", "\n\n");
 
         StringBuffer sb = new StringBuffer(text.length() + TEXT_TO_HTML_EXTRA_BUFFER_LENGTH);