Enhance documentation for MakeSweep

Describe explicitly what ranges for angles and color stops work and what
the behavior is for angles smaller than 0 and greater than 360.

Change-Id: I2e9f779e011b33ee012c8c3739b310ce7fe10602
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/568757
Reviewed-by: Florin Malita <fmalita@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Dominik Röttsches <drott@google.com>
1 file changed