_content/ref/mod: document Go 1.17 lazy loading and module graph pruning
_content/ref/mod: document Go 1.17 lazy loading and module graph pruning

For golang/go#36460

Change-Id: I36b3657103f069412b225356d011a19c1a10109e
Reviewed-on: https://go-review.googlesource.com/c/website/+/333629
Trust: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
Run-TryBot: Bryan C. Mills <1c8aad60184261ccede67f5c63a0d2a3bf3c9ff4@google.com>
TryBot-Result: Go Bot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Jay Conrod <f3fadaedc813344da5ec13b41e26596889cc5baa@google.com>
Reviewed-by: Michael Matloob <4562438b12c14e81a47550acde616ff328a909f8@golang.org>

golang/website,golang/website,golang/website,golang/website,golang/website,golang/website,golang/website