docs: clarify when APIs use context.Background.

The Go standard library retrofitted context support onto existing APIs
using context.Background and later offered variants that directly
supported user-defined context value specification. This commit makes
that behavior clear in documentation and suggests context-aware
alternatives if the user is looking for one.

An example motivation is supporting code for use in systems that expect
APIs to be cancelable for lifecycle correctness or load
shedding/management reasons, as alluded to in
https://blog.golang.org/context-and-structs.

Updates #44143

Change-Id: I2d7f954ddf9b48264d5ebc8d0007058ff9bddf14
Reviewed-on: https://go-review.googlesource.com/c/go/+/296152
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Jean de Klerk <deklerk@google.com>
Trust: Jean de Klerk <deklerk@google.com>
Run-TryBot: Jean de Klerk <deklerk@google.com>
TryBot-Result: Go Bot <gobot@golang.org>

tls.go

@@ -111,6 +111,9 @@ func (timeoutError) Temporary() bool { return true }
 //
 // DialWithDialer interprets a nil configuration as equivalent to the zero
 // configuration; see the documentation of Config for the defaults.
+//
+// DialWithDialer uses context.Background internally; to specify the context,
+// use Dialer.DialContext with NetDialer set to the desired dialer.
 func DialWithDialer(dialer *net.Dialer, network, addr string, config *Config) (*Conn, error) {
 	return dial(context.Background(), dialer, network, addr, config)
 }
@@ -224,6 +227,9 @@ type Dialer struct {
 // handshake, returning the resulting TLS connection.
 //
 // The returned Conn, if any, will always be of type *Conn.
+//
+// Dial uses context.Background internally; to specify the context,
+// use DialContext.
 func (d *Dialer) Dial(network, addr string) (net.Conn, error) {
 	return d.DialContext(context.Background(), network, addr)
 }