testing: clarify rules for concurrent t.Run calls

testing: clarify rules for concurrent t.Run calls

Apparently, "all such calls must happen" means that the t.Run call
must *return* before the outer test function returns, or the calls
will cause a data race on t.ran.

Clarify the docs.

Fixes #20339

Change-Id: I191a9af2a9095be1e0aaf10b79c30e00a9c495cb
Reviewed-on: https://go-review.googlesource.com/47150
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>