x/net/proxy: document returned Dialers implement ContextDialer

[sjpotter]

Currently, if one tries to instantiate a socks5 dialer, one receives a response with the proxy.Dialer interface. This interface only exposes the Dial() (context free) method. This method is deprecated in many cases (i.e. http client transports). DialContext() is actually implemented for all the proxies, but is only visible within the internal package, so is not actually usable by end users.

Motivated by golang/net#164 but the comment there is that since that PR changes the proxy.Dialer interface it breaks backwards compatibility (which I assume is if external users use the interface, and not just limited to the proxy package) and therefore should go through the proposal process to determine what API changes should be.

[seankhliao]

Interfaces can't be changed, that breaks backwards compatibility.

golang.org/x/net/proxy already has an interface for this ContextDialer

What can change is documentation to say that all returned dialers can be asserted to ContextDialer