Expand documentation of SkOpts
Expand documentation of SkOpts

I need to get the Bazel and G3 version working, so I had to figure out
how it worked. After doing so, I wrote it down.

Change-Id: Ibacb1952029582bfbea469a46f76ff3170c3be49
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/579365
Reviewed-by: Ben Wagner <53965154ffeecc4a10886eab9259e95965f2979a@google.com>
Reviewed-by: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia,google/skia