[release-branch.go1.19] cmd/go/internal: update documentation of go test and go generate

For #57050.
Fixes #60457.

Change-Id: I46cac667ff78ac171c878f4366f8f01f58f1d27d
GitHub-Last-Rev: 697c255
GitHub-Pull-Request: #57814
Reviewed-on: https://go-review.googlesource.com/c/go/+/461683
TryBot-Result: Gopher Robot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>
Reviewed-by: Bryan Mills <[email protected]>
(cherry picked from commit 93d9035)
Reviewed-on: https://go-review.googlesource.com/c/go/+/499295
Run-TryBot: Dmitri Shuralyov <[email protected]>
Auto-Submit: Dmitri Shuralyov <[email protected]>

Author: shogohida

src/cmd/go/alldocs.go

@@ -89,6 +89,11 @@ Go generate sets several variables when it runs the generator:
 		generator, containing the Go toolchain and standard library.
 	$DOLLAR
 		A dollar sign.
+	$PATH
+		The $PATH of the parent process, with $GOROOT/bin
+		placed at the beginning. This causes generators
+		that execute 'go' commands to use the same 'go'
+		as the parent 'go generate' command.
 
 Other than variable substitution and quoted-string evaluation, no
 special processing such as "globbing" is performed on the command

src/cmd/go/internal/generate/generate.go

@@ -90,6 +90,10 @@ standard output, even if the test printed them to its own standard
 error. (The go command's standard error is reserved for printing
 errors building the tests.)
 
+The go command places $GOROOT/bin at the beginning of $PATH
+in the test's environment, so that tests that execute
+'go' commands use the same 'go' as the parent 'go test' command.
+
 Go test runs in two different modes:
 
 The first, called local directory mode, occurs when go test is