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?

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.

---

Constantine Fry seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}