feat: add description for Client interface

[xuthus5]

Please do not merge requests until we have discussed the issue clearly.

Our open API should allow users to quickly understand how to use it, but at present, I still have misunderstandings about the definitions of ShowTagKeys and ShowSeries.I think the definitions of these APIs are very poor, ShowTagKeys, ShowTagValues, ShowFieldKeys, ShowSeries

1. The purpose are unclear
2. Input parameters command are unclear
3. The return value type is unclear

[shoothzj]

Can we add it to design doc first?

[shoothzj]

I think we should establish a mechanism to synchronize method descriptions, as they contain language-independent content.

[xuthus5]

impl by: #165 , close it.