encode.h: mention 'exact' default in WebPEncodeLossless*

note that config.exact defaults to 0 and point users to WebPEncode() if
the default isn't acceptable.

BUG=webp:424

Change-Id: I179c34649834aeadc1606d0856f33e8255048ea1

src/webp/encode.h

@@ -62,6 +62,10 @@ WEBP_EXTERN size_t WebPEncodeBGRA(const uint8_t* bgra,
 // These functions are the equivalent of the above, but compressing in a
 // lossless manner. Files are usually larger than lossy format, but will
 // not suffer any compression loss.
+// Note these functions, like the lossy versions, use the library's default
+// settings. For lossless this means 'exact' is disabled. RGB values in
+// transparent areas will be modified to improve compression. To avoid this,
+// use WebPEncode() and set WebPConfig::exact to 1.
 WEBP_EXTERN size_t WebPEncodeLosslessRGB(const uint8_t* rgb,
                                          int width, int height, int stride,
                                          uint8_t** output);