Notes that it does nothing if the combination of `signal`,
`callback`, and `data` is already connected to the handler.
Adds missing param `signal` description.
Typo fix
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.
Adds a simple signal reference counting function
(signal_handler_connect_ref) that makes it so that signals keep the
handler around until the all the signal itself is disconnected. This
prevents potential crashes where a signal might try to disconnect after
a handler has already been destroyed (typically in C++ with
OBSSignalHandler helper objects, where destruction isn't guaranteed to
be predictable).
This also modifies OBSSignalHandler to use the reference-counting
connections.
