Make sure it's documented that channel names etc. have to be URL-encoded in the rest api

[SimonWoolf]

Tom asked in slack about a rest api query that was failing because the channel name wasn't properly url-encoded. After replying I had a quick look through the spec and the rest api docs to find a reference for the channel name having to be url-encoded, but couldn't find anything. Might have just missed it, but if not, we should add it, in both places

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