Clarify the expected 'config' lifespan in WebPIDecode()
Clarify the expected 'config' lifespan in WebPIDecode()

Change-Id: I08c880adc34d651a63fa2d07cfb07b1468a6df1b

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