[canvaskit] Make build docs more clear

Bug: skia:10573
Change-Id: I4142418f0f26859a7953dbeeb8c25fa176719f76
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/328598
Reviewed-by: Kevin Lubick <kjlubick@google.com>
1 file changed