Improve documentation & editing support for WebRTC

[dermotduffy]

• Better clarify the limitations/requirements of the WebRTC "support" this card offers so it causes less confusion.
• Provide GUI editing support for the URL parameter (as well as the entity parameter).
• Closes "ERROR: Support only RTSP-stream" on all cameras when using WebRTC (but direct camera URLs work with WebRTC Card) #69

[github-actions]

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