net/http: mention RegisterOnShutdown in Shutdown docs

[artyom]

http.Server.RegisterOnShutdown was introduced in 1.9 cycle, but it's not mentioned in the release notes, so it's easy to miss, even if it's useful to you.

Documentation for http.Server.Shutdown (that was introduced in 1.8) states:

Shutdown does not attempt to close nor wait for hijacked connections such as WebSockets. The caller of Shutdown should separately notify such long-lived connections of shutdown and wait for them to close, if desired.

IMO this is a good place to mention the new RegisterOnShutdown method to increase awareness of its existence.

[odeke-em]

@artyom thank you for noticing and reporting this! Would you like to make a CL for this?

Meanwhile I'll ping @bradfitz @rsc @broady on whether we can retroactively update these docs.

[gopherbot]

Change https://golang.org/cl/59350 mentions this issue: net/http: mention RegisterOnShutdown in Server.Shutdown docs