pad: fix default acceptcaps
pad: fix default acceptcaps

Make the acceptcaps function behave like all the other functions with a default
implementation. Don't try to chain up to the default implementation when it was
set to NULL explicitly but return FALSE instead.
Fix some docs

Lachann/gstreamer,StreamUtils/gstreamer,lubosz/gstreamer,surround-io/gstreamer,cfoch/gstreamer,Lachann/gstreamer,cfoch/gstreamer,krichter722/gstreamer,jpakkane/gstreamer,mparis/gstreamer,lubosz/gstreamer,ensonic/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,cfoch/gstreamer,cfoch/gstreamer,jpakkane/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,jpxiong/gstreamer,cablelabs/gstreamer,shelsonjava/gstreamer,krieger-od/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,lubosz/gstreamer,cfoch/gstreamer,collects/gstreamer,mparis/gstreamer,lovebug356/gstreamer,lovebug356/gstreamer,krieger-od/gstreamer,krieger-od/gstreamer,centricular/gstreamer,jpxiong/gstreamer,shelsonjava/gstreamer,surround-io/gstreamer,justinjoy/gstreamer,jpakkane/gstreamer,krieger-od/gstreamer,jpakkane/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,Lachann/gstreamer,jpakkane/gstreamer,lovebug356/gstreamer,collects/gstreamer,justinjoy/gstreamer,justinjoy/gstreamer,centricular/gstreamer,shelsonjava/gstreamer,lovebug356/gstreamer,jpxiong/gstreamer,ensonic/gstreamer,collects/gstreamer,StreamUtils/gstreamer,surround-io/gstreamer,ensonic/gstreamer,ensonic/gstreamer,cablelabs/gstreamer,mparis/gstreamer,krichter722/gstreamer,centricular/gstreamer,collects/gstreamer,justinjoy/gstreamer,surround-io/gstreamer,justinjoy/gstreamer,krieger-od/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,lubosz/gstreamer,cablelabs/gstreamer,jpxiong/gstreamer,mparis/gstreamer,jpxiong/gstreamer,Lachann/gstreamer,Lachann/gstreamer,centricular/gstreamer,lovebug356/gstreamer,mparis/gstreamer,lubosz/gstreamer,centricular/gstreamer,StreamUtils/gstreamer,krichter722/gstreamer,collects/gstreamer