cmd/go/internal: add a note about GOCACHE=off

Fixes #25928

Change-Id: I1401ecc54af26eeeee648bb8eeb5d2d3566fa60c
Reviewed-on: https://go-review.googlesource.com/119695
Reviewed-by: Ian Lance Taylor <[email protected]>

Author: agnivade

src/cmd/go/alldocs.go

@@ -658,6 +658,7 @@ The default location for cache data is a subdirectory named go-build
 in the standard user cache directory for the current operating system.
 Setting the GOCACHE environment variable overrides this default,
 and running 'go env GOCACHE' prints the current cache directory.
+You can set the variable to 'off' to disable the cache.
 
 The go command periodically deletes cached data that has not been
 used recently. Running 'go clean -cache' deletes all cached data.