docs: update qos design doc
docs: update qos design doc

Fix some typos.
change the definition of the quality field for video decoders to something that
makes more sense.

drothlis/gstreamer,ensonic/gstreamer,krichter722/gstreamer,jpxiong/gstreamer,lubosz/gstreamer,ensonic/gstreamer,krieger-od/gstreamer,centricular/gstreamer,justinjoy/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,krichter722/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,StreamUtils/gstreamer,cablelabs/gstreamer,krieger-od/gstreamer,mparis/gstreamer,surround-io/gstreamer,Lachann/gstreamer,cablelabs/gstreamer,krieger-od/gstreamer,lubosz/gstreamer,magcius/gstreamer,StreamUtils/gstreamer,Lachann/gstreamer,mparis/gstreamer,ensonic/gstreamer,krichter722/gstreamer,Lachann/gstreamer,lovebug356/gstreamer,ahmedammar/platform_external_gst_gstreamer,lubosz/gstreamer,collects/gstreamer,ahmedammar/platform_external_gst_gstreamer,centricular/gstreamer,lovebug356/gstreamer,justinjoy/gstreamer,shelsonjava/gstreamer,collects/gstreamer,drothlis/gstreamer,mparis/gstreamer,jpakkane/gstreamer,centricular/gstreamer,Distrotech/gstreamer,ensonic/gstreamer,justinjoy/gstreamer,Distrotech/gstreamer,krieger-od/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,surround-io/gstreamer,krieger-od/gstreamer,centricular/gstreamer,cablelabs/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,mrchapp/gstreamer,Distrotech/gstreamer,jpakkane/gstreamer,jpakkane/gstreamer,jpxiong/gstreamer,ahmedammar/platform_external_gst_gstreamer,mrchapp/gstreamer,cfoch/gstreamer,ensonic/gstreamer,mrchapp/gstreamer,ylatuya/gstreamer,Lachann/gstreamer,lovebug356/gstreamer,justinjoy/gstreamer,jpxiong/gstreamer,centricular/gstreamer,ylatuya/gstreamer,ylatuya/gstreamer,lovebug356/gstreamer,jpxiong/gstreamer,StreamUtils/gstreamer,magcius/gstreamer,lovebug356/gstreamer,Distrotech/gstreamer,cfoch/gstreamer,magcius/gstreamer,ahmedammar/platform_external_gst_gstreamer,lubosz/gstreamer,magcius/gstreamer,drothlis/gstreamer,drothlis/gstreamer,mrchapp/gstreamer,magcius/gstreamer,mrchapp/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,drothlis/gstreamer,collects/gstreamer,collects/gstreamer,ahmedammar/platform_external_gst_gstreamer,shelsonjava/gstreamer,lubosz/gstreamer,cfoch/gstreamer,ylatuya/gstreamer,mparis/gstreamer,mparis/gstreamer,jpxiong/gstreamer,surround-io/gstreamer,ylatuya/gstreamer,Lachann/gstreamer,collects/gstreamer,shelsonjava/gstreamer,krichter722/gstreamer,justinjoy/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,cablelabs/gstreamer,Distrotech/gstreamer