Improve help text of the !clean subcommands

[camcaswell]

The !clean command has multiple modes, and it can be confusing to use.

• !clean bots mentions a traverse argument that doesn't exist.
• !clean regex has an example of formatting the regex pattern that I think needs more escape characters.
• Most of the commands can take either a message or a time limit. They should make it clear that providing a message limits the search to its channel, and to clean across channels you need to provide a time limit instead.

Anyone can implement

[Autonymic]

Hello, I'd like to try this issue as well, since it is related to the other one. Please assign me to it.