Update rounding/clamping comments on sk_linear_to_srgb().

Rounding is enough.  No need for an explicit clamp if the inputs are in range.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2161223002

Review-Url: https://codereview.chromium.org/2161223002
2 files changed