Make channel parameter description more clear and specific.

[constantine-fry]

It's not clear from channel parameter description that one can put there identifiers of channels, ims or groups.

[dblock]

@constantine-fry Slack said they will not be updating that repo until they hire someone. We don't have to wait. I've pulled JSON files from https://github.com/slackhq/slack-api-docs into https://github.com/dblock/slack-api-ref, which I intend to maintain and kept JSON files only. Could you please PR this change and anything else you have there?

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