doc/go1.17: clarify Modules changes

Writing CL 333629 clarified my thinking about the behavioral changes
associated with lazy loading. There are really two interrelated
changes — graph pruning, and lazy loading proper — that are both made
possible by the added redundancy in the go.mod file.

(I had initially approached the whole cluster of features as “lazy
loading” because that was the starting point for the design. Graph
pruning came into the picture when we looked at how to bound the
worst-case behavior of lazy loading, but it is really the more
important of the two aspects of the design.)

Note that this change adds links to doc anchors added in CL 333629.

Fixes #36460
Fixes #47397

Change-Id: I0ef4af57f647bf5ee210ea7099191fb4befa2cc1
Reviewed-on: https://go-review.googlesource.com/c/go/+/335135


Trust: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Jay Conrod <jayconrod@google.com>