sync bookmaker docs with canvas

removed SrcRectConstraint from docs to match SkCanvas.h
also removed constraint description paragraph from SkCanvas.h

TBR=bsalomon@google.com

Docs-Preview: https://skia.org/?cl=165821
Bug: skia:5679
Change-Id: I34c51f672477076f182946ce71e2d06f049e7f29
Reviewed-on: https://skia-review.googlesource.com/c/165821
Commit-Queue: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Cary Clark <caryclark@skia.org>
3 files changed