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>

src/cmd/go/alldocs.go

@@ -1953,6 +1953,13 @@
 // The -n flag causes tool to print the command that would be
 // executed but not execute it.
 //
+// The -modfile=file.mod build flag causes tool to use an alternate file
+// instead of the go.mod in the module root directory.
+//
+// Tool also provides the -C, -overlay, and -modcacherw build flags.
+//
+// For more about build flags, see 'go help build'.
+//
 // For more about each builtin tool command, see 'go doc cmd/<command>'.
 //
 // # Print Go version

src/cmd/go/internal/tool/tool.go

@@ -46,6 +46,13 @@ With no arguments it prints the list of known tools.
 The -n flag causes tool to print the command that would be
 executed but not execute it.
 
+The -modfile=file.mod build flag causes tool to use an alternate file
+instead of the go.mod in the module root directory.
+
+Tool also provides the -C, -overlay, and -modcacherw build flags.
+
+For more about build flags, see 'go help build'.
+
 For more about each builtin tool command, see 'go doc cmd/<command>'.
 `,
 }