add missing description of the alpha filtering methods
add missing description of the alpha filtering methods

Change-Id: Id7bdf5e30121ce5d610b7fa00f64bcd2d7bed2bf

kleopatra999/webm.libwebp,Acidburn0zzz/webm.libwebp,Aexyn/libwebp,gshORTON/webm.libwebp,kim42083/webm.libwebp,yohunl/libwebp,PKRoma/libwebp,iniwf/webm.libwebp,imazen/libwebp,ttyangf/libwebp,ya7lelkom/libwebp,comoyo/gonzo-libwebp,jacklicn/libwebp,kleopatra999/webm.libwebp,Aexyn/libwebp,appunite/libwebp,webmproject/libwebp,comoyo/gonzo-libwebp,cysp/libwebp,Acidburn0zzz/webm.libwebp,redengineer/libwebp,ericmckean/webm.libwebp,comoyo/gonzo-libwebp,altogother/webm.libwebp,zofuthan/libwebp,Acidburn0zzz/webm.libwebp,jiehu5114/libwebp,comoyo/gonzo-libwebp,PKRoma/libwebp,Maria1099/webm.libwebp,matanbs/webm.libwebp,Maria1099/webm.libwebp,Tekserve/libwebp,Suvarna1488/webm.libwebp,ttyangf/libwebp,gshORTON/webm.libwebp,imazen/libwebp,jiehu5114/libwebp,ericmckean/webm.libwebp,keane83/BLE_IM,redengineer/libwebp,zofuthan/libwebp,kalli123/webm.libwebp,appunite/libwebp,altogother/webm.libwebp,keane83/BLE_IM,ya7lelkom/libwebp,PKRoma/libwebp,iniwf/webm.libwebp,matanbs/webm.libwebp,abwiz0086/webm.libwebp,altogother/webm.libwebp,kim42083/webm.libwebp,matanbs/webm.libwebp,comoyo/gonzo-libwebp,ya7lelkom/libwebp,altogother/webm.libwebp,kleopatra999/webm.libwebp,appunite/libwebp,cysp/libwebp,dumganhar/libwebp,rxl194/libwebp,Tekserve/libwebp,Distrotech/libwebp,Suvarna1488/webm.libwebp,cysp/libwebp,jiehu5114/libwebp,jiehu5114/libwebp,Acidburn0zzz/webm.libwebp,kim42083/webm.libwebp,appunite/libwebp,rxl194/libwebp,webmproject/libwebp,kim42083/webm.libwebp,redengineer/libwebp,kalli123/webm.libwebp,abwiz0086/webm.libwebp,redengineer/libwebp,kleopatra999/webm.libwebp,rxl194/libwebp,Suvarna1488/webm.libwebp,ericmckean/webm.libwebp,yohunl/libwebp,abwiz0086/webm.libwebp,Maria1099/webm.libwebp,ya7lelkom/libwebp,dumganhar/libwebp,reimaginemedia/webm.libwebp,gshORTON/webm.libwebp,ericmckean/webm.libwebp,Suvarna1488/webm.libwebp,jacklicn/libwebp,comoyo/gonzo-libwebp,cysp/libwebp,rxl194/libwebp,jiehu5114/libwebp,imazen/libwebp,reimaginemedia/webm.libwebp,keane83/BLE_IM,matanbs/webm.libwebp,gshORTON/webm.libwebp,redengineer/libwebp,ericmckean/webm.libwebp,abwiz0086/webm.libwebp,appunite/libwebp,Suvarna1488/webm.libwebp,matanbs/webm.libwebp,Acidburn0zzz/webm.libwebp,kalli123/webm.libwebp,zofuthan/libwebp,Distrotech/libwebp,PKRoma/libwebp,ya7lelkom/libwebp,kalli123/webm.libwebp,dumganhar/libwebp,zofuthan/libwebp,abwiz0086/webm.libwebp,jacklicn/libwebp,rxl194/libwebp,reimaginemedia/webm.libwebp,kim42083/webm.libwebp,kalli123/webm.libwebp,reimaginemedia/webm.libwebp,PKRoma/libwebp,appunite/libwebp,imazen/libwebp,iniwf/webm.libwebp,ttyangf/libwebp,Maria1099/webm.libwebp,comoyo/gonzo-libwebp,PKRoma/libwebp,ttyangf/libwebp,kleopatra999/webm.libwebp,Aexyn/libwebp,gshORTON/webm.libwebp,Tekserve/libwebp,jacklicn/libwebp,reimaginemedia/webm.libwebp,altogother/webm.libwebp,yohunl/libwebp,Maria1099/webm.libwebp,ttyangf/libwebp,webmproject/libwebp,imazen/libwebp,Distrotech/libwebp,Aexyn/libwebp,Aexyn/libwebp,yohunl/libwebp,webmproject/libwebp,yohunl/libwebp,jacklicn/libwebp,zofuthan/libwebp,iniwf/webm.libwebp,webmproject/libwebp,iniwf/webm.libwebp,keane83/BLE_IM,keane83/BLE_IM,Tekserve/libwebp