testing: document that Skip cannot undo Error

Fixes #16502.

Change-Id: Id8e117a724d73cd51844c06d47bbeba61f8dc827
Reviewed-on: https://go-review.googlesource.com/31324Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>

src/testing/testing.go

@@ -520,6 +520,7 @@ func (c *common) Skipf(format string, args ...interface{}) {
 }
 
 // SkipNow marks the test as having been skipped and stops its execution.
+// If a test fails (see Error, Errorf, Fail) and is then skipped, it is still considered to have failed.
 // Execution will continue at the next test or benchmark. See also FailNow.
 // SkipNow must be called from the goroutine running the test, not from
 // other goroutines created during the test. Calling SkipNow does not stop