cmd/go/internal/cache: update docs on cache accessor func

Its docs went stale in Go 1.12 when the build cache became required.

Signed-off-by: Brad Fitzpatrick <bradfitz@golang.org>
Change-Id: Icb7b06dc5147a858f00460e7cfe85b79b6b6f0eb
Reviewed-on: https://go-review.googlesource.com/c/go/+/465555
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>

src/cmd/go/internal/cache/default.go

@@ -14,7 +14,8 @@ import (
 	"cmd/go/internal/cfg"
 )
 
-// Default returns the default cache to use, or nil if no cache should be used.
+// Default returns the default cache to use.
+// It never returns nil.
 func Default() *Cache {
 	defaultOnce.Do(initDefaultCache)
 	return defaultCache