gui: lots of functions lack what return value means.

[atsushieno]

Those functions in gui extension lacks documentation comment on what the return value means, for example in clap_plugin_gui :

• create()
• get_size()
• adjust_size()
• set_size()

It is not a comprehensive list. I think the entire specification should be reviewed and see if the overall documentation lacks description on parameters and return values, at least on non-draft ones.

[abique]

If those function return false, then it means they failed.

For example the GUI is on another computer, and your communication via the socket failed in get_size() so it returns false and the host should discard the with and height.

[niswegmann]

@abique, the function suggest_title in clap/ext/gui.h returns void and therefore shouldn't have the comment "Returns true on success.".