docs fix external substitutions

explict substitutions are required to distiguish words
that preserve underscores

R=caryclark@google.com

Docs-Preview: https://skia.org/?cl=134841
Bug: skia:6898
Change-Id: Id1a5669c180e03e048de3c0251882bd06b297283
Reviewed-on: https://skia-review.googlesource.com/134841
Reviewed-by: Cary Clark <caryclark@skia.org>
Commit-Queue: Cary Clark <caryclark@skia.org>
Auto-Submit: Cary Clark <caryclark@skia.org>
26 files changed