修复文档 paddle.nn.functional.embedding paddle.index_add

[cangtianhuang]

修复 paddle.nn.functional.embedding, issue：#7243

• 修改描述，将过时的 input、size 等参数替换为 x、weight
• 修复数值范围，改为 0 =< id < weight.shape[0]
• 删去 padding_idx 的 long 属性，参考：https://github.com/PaddlePaddle/Paddle/blob/develop/python/paddle/nn/functional/input.py#L164

修复 paddle.index_add, issue：#7225

• 修复代码示例
• 替换错误符号

[paddle-bot]

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[cangtianhuang]

You must have one TPM (jzhang533/ZhangJun or Ligoml/LiMengLiu or sunzhongkai588/SunZhongKai) approval for the api change! docs/api/paddle for the management reason of API interface and API document.

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@sunzhongkai588

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