#1587 Update plugin interface documentation

[gwon]

The function signature of xchat_plugin_init() is not defined. It is only available in the examples as:

int xchat_plugin_init(xchat_plugin *plugin_handle,
                  char **plugin_name,
                  char **plugin_desc,
                  char **plugin_version,
                  char *arg)

It should use const char** instead of char* so string literals can be used directly.

Also, there is no information about xchat_plugin_get_info().

According to this forum post:

"There isn't actually anything making use of xchat_plugin_get_info right now. At one point someone was going to write a patch to allow you to diable plugin instead of having to move them out of your xchat data dir. xchat_plugin_get_info would have been used to retrieve the information for filling in the columns in the Plugins and Scripts window. The only reason to add that right now is to be future proof in case someone gets around to implementing that feature."

So xchat_plugin_get_info() should be required.