This is a followup to CL 24747, where the package doc phrase
"Functions in this package replace occurrences of slash unless otherwise specified."
was removed. The phrase was originally added in CL 7310 together
with this explicit opt out statement for SplitList.

Remove it since it's no longer neccessary. This helps consistency.

Updates #16111.
Updates #10122.

Change-Id: Iba86de57c24100adecac9cb5892ce180126c0ea6
Reviewed-on: https://go-review.googlesource.com/25250Reviewed-by: Russ Cox <rsc@golang.org>