testing: clarify T.Name returns a distinct name of the running test

According to the discussion, it is clear that T.Name returns a
distinct name among all tests. However, there is no specification
of how sub-tests with the same specified test name are constructed.
This change only clarifies the uniqueness and the components of the
name without suggesting any explicit format of the returned name.

Fixes #46488

Change-Id: I6cebd419b69fb08d8646cb744a129548452042ef
Reviewed-on: https://go-review.googlesource.com/c/go/+/337392


Reviewed-by: Ian Lance Taylor <iant@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>