Doc: clarify the role of quality in WebPConfig.
Doc: clarify the role of quality in WebPConfig.

Change-Id: I13706847f6263b9bb91eaf3c9284dd2d42ea7a85

PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,PKRoma/libwebp,PKRoma/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp,webmproject/libwebp,PKRoma/libwebp,webmproject/libwebp