GitLab

Reworking the comments in path to call out how leading
empty elements are treated. Also updating filepath.Join
since it shared much of the wording from path.Join.

Updates #35655

Change-Id: I5b15c5d36e9d19831ed39e6bcc7f2fd6c1330033
Reviewed-on: https://go-review.googlesource.com/c/go/+/207797

Reviewed-by: Rob Pike <r@golang.org>