bin: improve docs a little
bin: improve docs a little

Mention that a DURATION message does not mean that one can safely query the
duration on a bin, that only works when the bin is prerolled.

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