net/http: mention NewRequestWithContext+Client.Do for custom contexts

Adds mentions of NewRequestWithContext and *Client.Do as prescriptions
for how to use a specified context.Context, to the docs of:
* (*Client).Get
* (*Client).Head
* (*Client).Post
* (*Client).PostForm
* Get
* Head
* Post
* PostForm

given that we can't remove those convenience functions, nor
change the method signatures, except for Go2.

Fixes #35562

Change-Id: I4859e6757e7f958c9067ac4ef15881cfba7d1f8d
Reviewed-on: https://go-review.googlesource.com/c/go/+/299610


Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Emmanuel Odeke <emmanuel@orijtech.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Damien Neil <dneil@google.com>