docs: add documentation for expr function expression support

[BQXBQX]

Checklist

• commit message follows commit guidelines
• documents are updated

Description of change

[BQXBQX]

在我完善 demo 时，遇到了一个问题， attr 属性貌似无法使用 spec 传入，无论是单测的例子还是官网的例子，好像 attr 的调用方法都是 函数式 调用形式。而且官网的 函数式 -> spec 例子中，会把 attr 中定义的属性提取到父对象中。例如 https://g2.antv.antgroup.com/examples/composition/space#space-layer ，所以我们还需要支持 attr 吗？

[hustcc]

在我完善 demo 时，遇到了一个问题， attr 属性貌似无法使用 spec 传入，无论是单测的例子还是官网的例子，好像 attr 的调用方法都是 函数式 调用形式。而且官网的 函数式 -> spec 例子中，会把 attr 中定义的属性提取到父对象中。例如 https://g2.antv.antgroup.com/examples/composition/space#space-layer ，所以我们还需要支持 attr 吗？

算是一个 bug，那就先移除支持吧，记得文档也修改一下。

[coveralls]

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Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 86.768%

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Totals
Change from base Build 14211352360:	0.0%
Covered Lines:	10733
Relevant Lines:	11989

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💛 - Coveralls