strings: clarify usage of Title and ToTitle

This is intended to help clear up confusion around the usage of the
Title and ToTitle functions. It includes a link to define title case
to distinguish it from upper case. It also includes an additional
example for the ToTitle function to showcase the difference in behavior
between it and the Title function.

Fixes #33302

Change-Id: I44e62962fb04d0d22966a39eda3a2d16de7a2291
Reviewed-on: https://go-review.googlesource.com/c/go/+/187825


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