Update SkSL docs (isOpaque, folding)
Update SkSL docs (isOpaque, folding)

- Remove the isOpaque parameter from all makeShader calls
- Add some folding to emphasize important parts of fiddles

Bug: skia:12643
Change-Id: Iaae5d61b63b409a8754b5144dc2baf5927a14848
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/506316
Reviewed-by: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>
Commit-Queue: Brian Osman <794c0b5534edf5601d88e1d41975d0262da12894@google.com>

aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,google/skia,aosp-mirror/platform_external_skia,aosp-mirror/platform_external_skia,google/skia,aosp-mirror/platform_external_skia,google/skia,google/skia,google/skia,google/skia,aosp-mirror/platform_external_skia