update SkTPin()

 - Reword the comment to explain how it differs from std::clamp().
 - Document and test an alternative form using std::{min,max}.

Change-Id: I3a97b98a15303478a5a7ff8d0536829f6d5f1586
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/327696
Reviewed-by: Chris Dalton <csmartdalton@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed