cmd/go: add tool meta-pattern to go help packages

For #48429

Change-Id: I6932853c3156a68d099a749431d15d2c37785649
Reviewed-on: https://go-review.googlesource.com/c/go/+/632555
Reviewed-by: Sam Thanawalla <[email protected]>
Auto-Submit: Sam Thanawalla <[email protected]>
Reviewed-by: Michael Matloob <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: ConradIrwin

src/cmd/go/alldocs.go

@@ -49,7 +49,7 @@ environment variable (For more details see: 'go help gopath').
 If no import paths are given, the action applies to the
 package in the current directory.
 
-There are four reserved names for paths that should not be used
+There are five reserved names for paths that should not be used
 for packages to be built with the go tool:
 
 - "main" denotes the top-level package in a stand-alone executable.
@@ -64,6 +64,8 @@ Go library.
 - "cmd" expands to the Go repository's commands and their
 internal libraries.
 
+- "tool" expands to the tools defined in the current module's go.mod file.
+
 Package names match against fully-qualified import paths or patterns that
 match against any number of import paths. For instance, "fmt" refers to the
 standard library's package fmt, but "http" alone for package http would not