cmd/go: document multiple conditions in TestScript

This functionality already exists but was undocumented. Related to
comments in CL 198797.

Change-Id: Icce40bd7c362423e6ed9c20673ce3de1311e5fd5
Reviewed-on: https://go-review.googlesource.com/c/go/+/200040
Run-TryBot: Jay Conrod <jayconrod@google.com>
Reviewed-by: Bryan C. Mills <bcmills@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/testdata/script/README

@@ -79,7 +79,9 @@ should only run when the condition is satisfied. The available conditions are:
    - [exec:prog] for whether prog is available for execution (found by exec.LookPath)
 
 A condition can be negated: [!short] means to run the rest of the line
-when testing.Short() is false.
+when testing.Short() is false. Multiple conditions may be given for a single
+command, for example, '[linux] [amd64] skip'. The command will run if all conditions
+are satisfied.
 
 The commands are: