net/http: ProxyFromEnvironment documentation is wrong

[dfawley]

After the change in #40909 (referenced via #45227), the documentation for ProxyFromEnvironment still incorrectly claims:

HTTPS_PROXY takes precedence over HTTP_PROXY for https requests.

It seems HTTP_PROXY is no longer used at all for https requests. The documentation here should have been fixed when the breaking change was made in Go 1.16.

[mknyszek]

CC @neild

[gopherbot]

Change https://go.dev/cl/428775 mentions this issue: net/http: clarify proxy selection from environment

[gopherbot]

Change https://go.dev/cl/428795 mentions this issue: http/httpproxy: remove comment on https proxy precedance