If I run /join (without arguments) I currently get:

/join chan1[,chan2...]

It's unclear what this is suggesting. I think it should at least mention that the usage is incorrect, and the correct usage would look like what's shown.

Secondly, I think /join without an argument could join the channel for the current tab. The use case is this: suppose my nick is use so tiny automatically picks osa1_, but osa1_ is not registered, so I can't join +r channels, but I may have tabs to such channels from before the connection is lost. In these cases after changing my nick to osa1 I should be able to switch to those channels and just do /join instead of /join #channel_name and try to join the channel of the tab.