add the describe of the profile graph

[hongliuliao]

What problem does this PR solve?

Issue Number: NONE

Problem Summary: make brpc user know the graph means

What is changed and the side effects?

Changed: NONE

Side effects: NONE

• Performance effects(性能影响): NONE

• Breaking backward compatibility(向后兼容性): NONE

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Check List:

• Please make sure your changes are compilable(请确保你的更改可以通过编译).
• When providing us with a new feature, it is best to add related tests(如果你向我们增加一个新的功能, 请添加相关测试).
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