doc : fixing missing @return in nvim_open_term doc

[michaelb]

Stumbled on this while developping a plugin, and while it definitively return a channel ID I was able to use, it's not documented ^^

@vigoux here you are :-)

[vigoux]

@bfredl i sundenly have a doubt, do we document return values on error, as the error will be returned anyway?

[michaelb]

@bfredl i sundenly have a doubt, do we document return values on error, as the error will be returned anyway?

Looks like some of the functions with sensible error handling do that: https://neovim.io/doc/user/api.html#nvim_create_buf()