Ensure all internal structs that are exposed publically are fully documented

[yuandrew]

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There are a number of internal structs that are exposed publicaly, but have missing docs for their fields, like

sdk-go/internal/activity.go

Line 27 in 6c2f815

ActivityInfo struct {

Describe the solution you'd like
Should go through and ensure these are all documented, this mainly won't require writing new docs, but simply copying other instances of usages of it's docs, saving developers from having to search the codebase to find docs for the same field in a different struct.

It would also be nice to enhance the doclink tool to identify and enforce this

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