go.dev: add information on Go version of standard library symbols

[danielfurman]

What is the URL of the page with the issue?

Any standard library package page, e.g. https://pkg.go.dev/net/http?tab=doc#Request.Clone

What is your user agent?

Mozilla/5.0 (Macintosh; Intel Mac OS X 10.15; rv:75.0) Gecko/20100101 Firefox/75.0

Screenshot

What did you do?

I navigated to the Request.Clone() method introduced in Go 1.13.

What did you expect to see?

Information in which Go version the method was introduced. Such information is available at https://golang.org/pkg/net/http/#Request.Clone:

What did you see instead?

No information in which Go version the method was introduced.

[danielfurman]

The same situation with fields of types, e.g. https://pkg.go.dev/net/http?tab=doc#Transport.ForceAttemptHTTP2 has no Go 1.13 label.

[julieqiu]

This is similar to #37102. Also related: #37819

/cc @fflewddur @Joanne881107

[dmitshur]

This is similar to #37102.

@julieqiu Should we close this as a duplicate of #37102, or is there something different about this issue?

[julieqiu]

I think it makes sense to close this as a duplicate. @Fenthick feel free to comment on #37102 with anything else.