docs: Clarify some aspects of OBS_PROPERTY_BUTTON

The important point here is that "data" from the callback isn't defined
by libobs, but rather by the API consumer.
Additionally, it's not entirely clear from the documentation whether the
callback would still be called for button of type OBS_BUTTON_URL. There
should be no negatives to always calling it and the OBS Studio frontend
already behaves that way, so let's document that in libobs.

docs/sphinx/reference-properties.rst

@@ -270,7 +270,11 @@ Property Object Functions
 
    :param    name:        Setting identifier string
    :param    text:        Localized name shown to user
-   :param    callback:    Callback to be executed when the button is pressed
+   :param    callback:    Callback to be executed when the button is pressed. Note that if the property
+                          is created with :c:func:`obs_properties_add_button` instead of
+                          :c:func:`obs_properties_add_button2`, the value of ``data`` is determined by
+                          the caller of :c:func:`obs_property_button_clicked`, and as such unspecified
+                          by libobs.
    :param    priv:        Pointer passed back as the `data` argument of the callback
    :return:               The property
 
@@ -642,6 +646,16 @@ Property Modification Functions
 
 .. function:: bool obs_property_button_clicked(obs_property_t *p, void *obj)
 
+   Calls the ``callback`` of the button.
+
+   :param p:   The property
+   :param obj: Must be a pointer to an object of type ``obs_canvas_t``, ``obs_source_t``,
+               ``obs_output_t``, ``obs_encoder_t``, or ``obs_service_t``.
+               If the property was created with :c:func:`obs_properties_add_button`, that
+               object's context will be passed as the ``obj`` parameter.
+               If the property was created with :c:func:`obs_properties_add_button2`, it
+               will be ignored.
+
 ---------------------
 
 .. function:: void obs_property_set_visible(obs_property_t *p, bool visible)
@@ -813,6 +827,8 @@ Property Modification Functions
 
 .. function:: void obs_property_button_set_type(obs_property_t *p, enum obs_button_type type)
 
+   Sets the type of the button. The button's ``callback`` will be called regardless of type.
+
    :param   type: Can be one of the following values:
 
                   - **OBS_BUTTON_DEFAULT** - Standard button
@@ -822,3 +838,6 @@ Property Modification Functions
 ---------------------
 
 .. function:: void obs_property_button_set_url(obs_property_t *p, char *url)
+
+   :param p:   The property
+   :param url: The URL to be opened if the button is of type ``OBS_BUTTON_URL``