Modify gas limit documentation

[darwintree]

Describe the feature or improvement 描述新特性或改进

After last hardfork, the meaning of the gas limit field returned from block changes. This field value will be modified in a recent node update so the document should change accordingly

Optional: Affected page(s) or section(s) 受影响的页面

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Optional: Why is this feature or improvement important? 为什么需要这个新特性或改进？

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Optional: Implementation ideas 如何实现

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Optional: Additional context 其他上下文信息

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Contribution Confirmation

• I'd be willing to contribute this feature myself within 2 months. 我将在两个月内自己实现这个feature