strings,bytes,regexp: use lists in Split* docstrings

This looks better than the default of using a code block.
While at it, fix punctuation.

Change-Id: I86abca4da1e9999b7e9043e615ad0988d35a5a46
Reviewed-on: https://go-review.googlesource.com/c/go/+/597656


Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>