cmd/go: document -modfile and other flags for 'go tool'

cmd/go: document -modfile and other flags for 'go tool'

Mention -modfile, -C, -overlay, and -modcacherw in the 'go tool'
documentation. We let a reference to 'go help build' give a pointer to
more detailed information.

The -modfile flag in particular is newly useful with the Go 1.24 support
for user-defined tools with 'go tool'.

Updates #48429
Updates #33926
Updates #71663
Fixes #71502

Change-Id: Ida67df50ff774a0886733d661a40e27c2cadc0f6
Reviewed-on: https://go-review.googlesource.com/c/go/+/648577
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Conrad Irwin <conrad.irwin@gmail.com>
Reviewed-by: Michael Matloob <matloob@golang.org>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>
Auto-Submit: Sam Thanawalla <samthanawalla@google.com>