增加源码注释

除了使用频率极高的几个类JSON，JSONObject，JSONArray外，其余的源码几乎没有注释，甚至有大量的类一行注释也没有。希望能在这方面加强一下

The issue requests broader source comments beyond JSONObject and JSONArray but names no files, classes, tests, or entry points. Start by surveying the Java source tree and existing comments, then agree on a specific class or bounded group before making changes. Done should mean the selected source area has useful, consistent comments and its tests still pass.

Written by the indexing model from the issue text.