os/user: note in doc that user.Current is being cached

user.Current caches the current user after its first call, so changes to
the uid after the first call will not affect its result. As this might
be unexpected, it should be mentioned in the docs.

Fixes #27659

Change-Id: I8b3323d55441d9a79bc9534c6490884d8561889b
Reviewed-on: https://go-review.googlesource.com/136315


Reviewed-by: Ian Lance Taylor <iant@golang.org>