[Toolchain] Corresponding Flutter methods may not have return values at all

[yamasite]

Problem

Quite a few methods are not consistent in return values:

C++: public abstract int addPublishStreamUrl(String url, boolean transcodingEnabled);
Flutter: Future<void> addPublishStreamUrl(String url, bool transcodingEnabled);

C++ has a return value, but Flutter does not.

Solution

To ensure that Flutter doc does not generate return values in docs or comments, new attributes need to be added.

Need to update

<section id="return_values">
   <title>返回值</title>
   <ul>
       <li>0: 方法调用成功。</li>
       <li>&lt; 0: 方法调用失败。<ul>
       <li><codeph>ERR_INVALID_ARGUMENT</codeph> (2): 参数无效，一般是 URL 为空或是长度为 0 的字符串。</li>
       <li><codeph>ERR_NOT_INITIALIZED</codeph> (7): 推流时未初始化引擎。</li>
   </ul>
       </li>
   </ul>
</section>

to

<section id="return_values" props="native electron unity">
   <title>返回值</title>
   <ul>
       <li>0: 方法调用成功。</li>
       <li>&lt; 0: 方法调用失败。<ul>
       <li><codeph>ERR_INVALID_ARGUMENT</codeph> (2): 参数无效，一般是 URL 为空或是长度为 0 的字符串。</li>
       <li><codeph>ERR_NOT_INITIALIZED</codeph> (7): 推流时未初始化引擎。</li>
   </ul>
       </li>
   </ul>
</section>

[yamasite]

@wangjie-agora This change affects 180 api_ dita files