xerrors: undeprecate Errorf
xerrors: undeprecate Errorf

Message about deprecation of `xerrors.Errorf` may confuse people because
they will think that `fmt.Errorf` will work the same way. In fact,
fmt.Errorf will not capture a stack backtrace.

Change-Id: I912f6b2ef289e97e69fe21160af62ba1ef4aa0a5
GitHub-Last-Rev: 16289f933ca8f83195e309c932b58850053c1504
GitHub-Pull-Request: golang/xerrors#3
Reviewed-on: https://go-review.googlesource.com/c/xerrors/+/410314
Reviewed-by: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
Reviewed-by: Cherry Mui <d62e63aa42ce272d7b6a5055d97e942b33a34679@google.com>
Auto-Submit: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Run-TryBot: Ian Lance Taylor <87e9c6d529889242b7e184afb632328636553ab4@google.com>

golang/xerrors