time: clarify Equal docs

time: clarify Equal docs

The docs used to imply that using == would compare Locations, but of
course it just compares Location pointers, which will have unpredictable
results depending on how the pointers are loaded.

Change-Id: I783c1309e476a9616a1c1c290eac713aba3b0b57
Reviewed-on: https://go-review.googlesource.com/32332
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>