Document global options

timeout is a global option that can be passed to the CLI and to the HTTP API. It is referenced in a few places, like in ipfs diag net --help, but it isn't clear there that it is global, and it may seem that it is only relevant for those subcommands (like diag net). There should be a mention of global options, somewhere in the CLI man (and in the HTTP API docs, but we can get to that later).

The obvious place would be at the bottom of ipfs --help. What do you think?