man cwebp: add precision about exactness of the 'lossless' mode
man cwebp: add precision about exactness of the 'lossless' mode

Since now, -exact is no longer the default.

Change-Id: I4f6ee835738decd01b0be03caabc1479750548a8

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