Adds links to release functions for "new/incremented reference".
Extra info on signal handler.
Parameter description on `obs_property_list_add_*` functions, since
their parameter `name` can be confusing and it is unclear which value
is stored in obs_data_t.
Adds clarifications on calldata_ptr.
Implements obs_data_get_defaults and updates the documentation. This is
supposed to allow someone to access all the defaults of an object.
Should help in cases where the full data is needed, and not just the
currently set.
