The callback types were not defined in the documentation. Since they are only
referenced once in the documentation, and they are not exported in hexchat-plugin.h,
we might as well "inline" them.
Added new functions to the plugin documentation:

  * hexchat_hook_server_attrs()
  * hexchat_hook_print_attrs()
  * hexchat_emit_print_attrs()
  * hexchat_event_attrs_create()
  * hexchat_event_attrs_free()
Merge pull request #7 from orium/doc-attrs-functs

Event attribute related documentation