runtime/debug: more complete BuildInfo documentation

runtime/debug: more complete BuildInfo documentation

A potential user did not realize Deps included all transitive dependencies,
not just direct dependencies of the main module. Clarify that and add
various other useful information.

Change-Id: I5b8e1314bb26092edbcc090ba8eb9859f0a70662
Reviewed-on: https://go-review.googlesource.com/c/go/+/453602
Run-TryBot: Russ Cox <rsc@golang.org>
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Julie Qiu <julieqiu@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Bryan Mills <bcmills@google.com>