Updated help text for WebRTC screen sharing

...urces/prod/help/images/ScreenSharingPresentersPreview.png

...nt/resources/prod/help/images/screenshare-chrome-stop.png

...client/resources/prod/help/images/screenshare-chrome1.png

...client/resources/prod/help/images/screenshare-chrome2.png

...lient/resources/prod/help/images/screenshare-chrome2a.png

...lient/resources/prod/help/images/screenshare-firefox1.png

...n-client/resources/prod/help/images/screenshare-step1.png

...n-client/resources/prod/help/webrtc-screenshare-help.html