cmd/go: document the build cache as safe for concurrent use

Fixes #26677

Change-Id: I2ca0408503000ccaddb0bd1fd359381ddd4fb699
Reviewed-on: https://go-review.googlesource.com/c/go/+/632895
Reviewed-by: Michael Matloob <matloob@golang.org>
Auto-Submit: Sam Thanawalla <samthanawalla@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Sam Thanawalla <samthanawalla@google.com>

src/cmd/go/alldocs.go

@@ -2264,6 +2264,7 @@
 // The go command caches build outputs for reuse in future builds.
 // The default location for cache data is a subdirectory named go-build
 // in the standard user cache directory for the current operating system.
+// The cache is safe for concurrent invocations of the go command.
 // Setting the GOCACHE environment variable overrides this default,
 // and running 'go env GOCACHE' prints the current cache directory.
 //

src/cmd/go/internal/help/helpdoc.go

@@ -809,6 +809,7 @@ var HelpCache = &base.Command{
 The go command caches build outputs for reuse in future builds.
 The default location for cache data is a subdirectory named go-build
 in the standard user cache directory for the current operating system.
+The cache is safe for concurrent invocations of the go command.
 Setting the GOCACHE environment variable overrides this default,
 and running 'go env GOCACHE' prints the current cache directory.