[SkPathBuilder] Add missing docstrings and tests

Bug: b/40040287
Change-Id: I4ac9476ba2116caacf44af1e0a426927ce3125f4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/994636
Reviewed-by: Florin Malita <fmalita@google.com>
Commit-Queue: Eric Boren <borenet@google.com>
2 files changed