Added a comment explaining DSL Declare's signature
Added a comment explaining DSL Declare's signature

Change-Id: Ie151d31f7662586862b17d994ccf196b1d492a68
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/406201
Commit-Queue: Ethan Nicholas <d2cfb8c2f603266f4dc3a6f09acbcc0089603b37@google.com>
Reviewed-by: John Stiles <f4fcf42d3bb5924557f1eeb3be66747535e585da@google.com>

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