Update rounding/clamping comments on sk_linear_to_srgb().

src/core/SkSRGB.h

 /*
  * Copyright 2016 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #ifndef SkSRGB_DEFINED
 #define SkSRGB_DEFINED
 
 #include "SkNx.h"
 
 /** Components for building our canonical sRGB -> linear and linear -> sRGB transformations.
  *
  *  Current best practices:
  *      - for sRGB -> linear, lookup R,G,B in sk_linear_from_srgb;
- *      - for linear -> sRGB, call sk_linear_to_srgb() for R,G,B, clamp to 255, and round;
+ *      - for linear -> sRGB, call sk_linear_to_srgb() for R,G,B, and round;
  *      - the alpha channel is linear in both formats, needing at most *(1/255.0f) or *255.0f.
  *
  *  sk_linear_to_srgb()'s output requires rounding; it does not round for you.
  *
  *  Given inputs in [0,1], sk_linear_to_srgb() will not underflow 0 but may overflow 255.
- *  The overflow is small enough that you can safely either clamp then round or round then clamp.
- *  (If you don't trust the inputs are in [0,1], you'd better clamp both sides immediately.)
+ *  The overflow is small enough to be handled by rounding.
+ *  (But if you don't trust the inputs are in [0,1], you'd better clamp both sides immediately.)
  *
  *  sk_linear_to_srgb() will run a little faster than usual when compiled with SSE4.1+.
  */
 
 extern const float sk_linear_from_srgb[256];
 
 static inline Sk4f sk_linear_to_srgb(const Sk4f& x) {
     // Approximation of the sRGB gamma curve (within 1 when scaled to 8-bit pixels).
     // For 0.00000f <= x <  0.00349f,    12.92 * x
     // For 0.00349f <= x <= 1.00000f,    0.679*(x.^0.5) + 0.423*x.^(0.25) - 0.101
     // Note that 0.00349 was selected because it is a point where both functions produce the
     // same pixel value when rounded.
     auto rsqrt = x.rsqrt(),
          sqrt  = rsqrt.invert(),
          ftrt  = rsqrt.rsqrt();
 
     auto lo = (12.92f * 255.0f) * x;
 
     auto hi = (-0.101115084998961f * 255.0f) +
               (+0.678513029959381f * 255.0f) * sqrt +
               (+0.422602055039580f * 255.0f) * ftrt;
 
     return (x < 0.00349f).thenElse(lo, hi);
 }
 
 #endif//SkSRGB_DEFINED