docs(Eino): update "fan-in and merge"

[wolfogre]

What type of PR is this?

docs: Documentation only changes

Check the PR title.

• [x] This PR title match the format: <type>(optional scope): <description>
• [x] The description of this PR title is user-oriented and clear enough for others to understand.

(Optional) Translate the PR title into Chinese.

更新”扇入与合并“一节。

(Optional) More detailed description for this PR(en: English/zh: Chinese).

en:

Follow the updates in https://github.com/cloudwego/eino/pull/137

zh(optional):

跟进 https://github.com/cloudwego/eino/pull/137 中的修改。

(Optional) Which issue(s) this PR fixes:

None.

[CLAassistant]

All committers have signed the CLA.

[GuangmingLuo]

Wait for: https://github.com/cloudwego/eino/issues/132

[wolfogre]

https://github.com/cloudwego/eino/issues/132 has been finished and the conflicts have been resolved.

[meguminnnnnnnnn]

@wolfogre 英文文档也辛苦加下吧

[wolfogre]

@meguminnnnnnnnn

Got it. ~But please hold on, I noticed that https://github.com/cloudwego/eino/issues/132 is not finished yet.~

~I tested eino v0.3.26 and found that there are some problems with ⁠RegisterValuesMergeFunc in certain extreme scenarios.~

[wolfogre]

Done.