add DEPRECATED labels on godoc

[shogo82148]

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godoc accepts “Deprecated:” comments as a depected feature.

https://go.dev/blog/godoc

Sometimes a struct field, function, type, or even a whole package becomes redundant or unnecessary, but must be kept for compatibility with existing programs. To signal that an identifier should not be used, add a paragraph to its doc comment that begins with “Deprecated:” followed by some information about the deprecation.

For example, https://pkg.go.dev/net/http/httputil#ClientConn